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Demography and public health
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Publisher: OxfordUniversityPress PrintPublicationDate: Feb2015PrintISBN-13: 9780199661756 Publishedonline: Feb2015DOI: 10.1093/med/9780199661756.001.0001
Chapter: DemographyandpublichealthAuthor(s): EmilyGrundyandMichaelMurphyDOI: 10.1093/med/9780199661756.003.0126
OxfordMedicine
OxfordTextbookofGlobalPublicHealth(6ed.)EditedbyRogerDetels,MartinGulliford,QuarraishaAbdoolKarim,andChorhChuanTan
Demographyandpublichealth
Introductiontodemographyandpublichealth
Thehealthandhealthcareneedsofapopulationcannotbemeasuredormetwithoutknowledgeofitssizeandcharacteristics.Demographyisconcernedwiththisandwithunderstandingpopulationdynamics—howpopulationschangeinresponsetotheinterplaybetweenfertility,mortality,andmigration.Thisunderstandingisaprerequisiteformakingtheforecastsaboutfuturepopulationsizeandstructurewhichshouldunderpinhealthcareplanning.Suchanalysesnecessitateareviewofthepast.Thenumberofveryoldpeopleinapopulation,forexample,dependsonthenumberofbirthseightorninedecadesearlierandrisksofdeathatsuccessiveagesthroughouttheinterveningperiod.Theproportionofveryoldpeopledependspartlyonthisnumeratorbutmoreimportantlyonthedenominator,thesizeofthepopulationasawhole.Thenumberofbirthsinapopulationdependsoncurrentpatternsoffamilybuilding,andalsoonthenumberofwomen‘atrisk’ofreproduction—itselfafunctionofpasttrendsinfertilityandmortality.Similarly,thenumberandcausesofdeathsarestronglyinfluencedbyagestructure.
Demography and public health
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Demographyislargelyconcernedwithansweringquestionsabouthowpopulationschangeandtheirmeasurement.Thebroaderfieldofpopulationstudiesembracesquestionsofwhythesechangesoccur,andwithwhatconsequences.
Thischapterpresentsinformationondemographicmethodsanddatasourcesandtheirapplicationtohealthandpopulationissues,togetherwithinformationondemographictrendsandtheirimplicationsandthemajortheoriesaboutdemographicchange,inordertoelucidatethecomplexinterrelationshipbetweenpopulationchangeandhumanhealth.
Globalissues
Fig.6.3.1showsthattheworld’spopulationhasrecentlybeengrowingatanunprecedentedrateandwasestimatedtobe7.05billionatmid2012(UN2011).Whileittookanestimated123years(from1804to1927)fortheworldtoincreaseitspopulationfrom1to2billion,theincreasefrom6to7billionwasachievedinatenthofthetime(1999–2011).TheUnitedNation’s(UN’s)mediumprojectionsuggestsafurtherincreaseofsome2.3billionby2050(UN2011).Beyondthis,thereisagoodchancethatglobalpopulationgrowthwillceasebytheendofthetwenty-firstcentury(LutzandSamir2010).
Fig.6.3.1Populationandprojectedpopulationoftheworldandmore,less,andleastdevelopedregions,1950–2050.
Source:datafromUnitedNations,WorldPopulationProspects:The2010Revision,UnitedNations,NewYork,USA,Copyright©2011,availablefromhttp://esa.un.org/unpd/wpp/Excel-Data/population.htm.
Thisprospectofglobalpopulationstabilitymaskshugedifferencesbetweenregionsandbetweenricherandpoorercountries.Between1950and2000,77percentofworldpopulationgrowthoccurredincountriescurrentlydesignatedbytheUNaslessdeveloped(excludingthe
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leastdeveloped,seeBox6.3.1fordefinitions);13percentinleastdevelopedcountriesand11percentinmoredevelopedregions.Between2000and2050,medium-termprojectionssuggestthatpopulationgrowthinmoredevelopedregionswillaccountforonly4percentofthetotalwith63percentoccurringinlessdevelopedcountriesand33percentintheleastdevelopedcountries.Theseprojectionsimplythatby2050theshareoftheworld’spopulationlivingincurrentlymoredevelopedregionswillaccountforonly14percentofthetotalworldpopulation—comparedwith32percentacenturyearlier—whiletherepresentationofthoseinthepoorestcountrieswillhaveincreasedfrom8percentofthetotalin1950to19percentin2050.
Box6.3.1Countryandregionalclassificationsbylevelofdevelopment
TheUNclassifiescountriesinto‘more’and‘less’developedandalsoidentifiesagroupof50‘leastdeveloped’countries.ThemoredevelopedcategoryincludesallofEurope,NorthAmerica,Australia,NewZealand,andJapan.Theleastdevelopedcountriesaremostlyinsub-SaharanAfricabutalsoincludeAfghanistan,Bangladesh,Cambodia,andMyanmar.TheclassificationhassomeanomaliesinthatsomewealthyAsianandNearEasterncountriesarecountedaslessdeveloped(e.g.SouthKorea,Singapore,Cyprus,Israel)whereassomepoorerformerEasternbloccountriesaretreatedasmoredeveloped(e.g.Albania,Belarus,Bulgaria).
TheWorldBankemploysaclassificationbasedongrossnationalincomepercapitawhichdividescountriesintohigh-,middle-,andlow-incomegroups,withasubdivisionofthemiddleintoupperandlower.Someofthecountries(principallyfromEasternEurope)classifiedbytheUNasdevelopedfallintomiddle-incomecategories,whilesomeoftheUNlessdevelopedgroupareclassifiedbytheWorldBankasmiddleincome(principallyLatinAmerican)orhighincome(someSouthEastAsian).
MembershipoftheOrganisationforEconomicCooperationandDevelopment(OECD)isalsosometimesusedasanindicatorofdevelopedcountrystatus;membersincludeRussiaandMexico,bothofwhichareclassifiedbytheWorldBankasmiddle-ratherthanhigh-incomecountries.
TheHumanDevelopmentIndexcompiledbytheUNDevelopmentProgrammetakesintoaccountfactorsotherthanincome,suchasschoolenrolment,literacy,andlevelsofmortality.
Regionalgroupingsemployedbydifferentinternationalagenciesalsovaryslightly.Furtherdetailsofalltheseclassificationsareavailableontherelevantorganizations’websites.
Whilesomeregionsgrapplewiththeneedsofrapidlygrowingpopulations,suchaslargeincreasesinrequirementsforchildhealthservicesandschools,othersfacechallengesofpopulationageingand,insomecases,populationdecline.By2025,nearlyaquarteroftheWesternEuropeanpopulationisexpectedtobeaged65ormoreandinsomecountries,suchasJapan,SouthKorea,Spain,andItaly,projectionssuggestthatathirdormoreofthepopulationwillbeaged65andoverby2050(UN2011).
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Thesehugelydifferingratesofgrowtharisefromdifferencesinvitalrates,andassociatedlargevariationsinagestructures,whichareillustratedforregionsandselectedcountrieswithintheminTable6.3.1.InanumberofEuropeancountriesandsomeAsiancountries,suchasJapan,womenonaveragehaveonly1.4childrenorfewer(seeBox6.3.2forderivationoftotalfertilityrate),andpeopleaged65andoveroutnumberchildrenunder15.Insub-SaharanAfrica,womenonaveragehavefivechildreneach,40percentormoreofthepopulationisaged15orunder,andonly3percentaged65ormore.
Table6.3.1Indicatorsofagestructure,fertility,andmortality:worldregionsandselectedcountries,2011
Region/country Proportion(%)ofpopulationaged:
Totalfertilityrate
Lifeexpectancyatbirth(years)
<15 65andover
Africa 41 3 4.6 58
Sub-Saharan
43 3 5.0 56
Northern 33 5 3.1 70
Asia 25 7 2.2 70
India 30 5 2.6 67
China 18 9 1.5 75
Japan 14 23 1.4 84
Indonesia 27 6 2.3 71
SouthKorea
16 11 1.2 79
Australia 18 14 1.8 82
Europe 15 16 1.6 76
Italy 14 20 1.4 82
Poland 15 14 1.3 76
Germany 13 21 1.4 80
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Germany 13 21 1.4 80
Sweden 15 20 1.7 81
Ukraine 14 15 1.3 69
UK 17 16 1.9 80
LatinAmericaandCaribbean
27 7 2.2 74
Brazil 25 7 1.8 73
Chile 22 9 1.9 78
Guatemala 38 4 3.3 71
NorthAmerica 19 14 2.0 79
UnitedStates
20 13 2.1 78
World 26 8 2.4 68
Source:datafromUnitedStatesCensusBureau,PopulationDivision,InternationalProgramsCenter,InternationalDataBase,availablefromhttp://www.census.gov/population/international/data/idb/informationGateway.php.
Levelsofmortality,andassociateddifferencesinageandcausedistributionofdeath,alsovarymarkedly.Insomehigh-incomecountries,averagelifeexpectancyatbirthisabove80,whileinsomesub-Saharancountriesitisbelow50,substantiallybecauseofHIV/AIDS.AsshowninFig.6.3.2,inSierraLeone,40percentofalldeathsinayearareofinfantsandchildrenagedunder5,comparedwith0.3percentinJapan.Conversely,ofalldeaths,70percentinJapanand50percentinChile,amiddle-incomecountry,areofpeopleaged75andover;equivalentproportionsforEgyptandSierraLeoneare20percentand8percentrespectively.Thesevariationshaveenormousimplicationsforhealthandhealthcareprioritiesin,andbeyond,thepopulationsconcerned.Divergenceinpopulationgrowthbetweenregionsoftheworldisalsofuellingmassmigration,whichitselfhasimplicationsforglobalpopulationhealth(Fernandesetal.2007).
Demography and public health
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Fig.6.3.2Cumulativedistributionofdeathsbyage;SierraLeone2009,Egypt2009,Chile2008,Japan2009.
Source:datafromUnitedNationsDepartmentofEconomicandSocialAffairs,DemographicYearbook2011,ST/ESA/STAT/SER.R/41,UnitedNations,NewYork,USA,Copyright©UnitedNations2012,availablefromhttp://unstats.un.org/unsd/demographic/products/dyb/dybsets/2011.pdf
Closelyrelatedtovariationsinthedistributionofdeathsbyagearedifferencesinthecausestructureofdeath.AsshowninTable6.3.2,communicablediseases,maternalandperinatalconditions,andnutritionaldeficienciesaccountfor67percentofalldeathsinsub-SaharanAfricabutonly5percentinEurope.Conversely,non-communicablediseasesareresponsiblefor25percentofdeathsinsub-SaharanAfrica,66percentinAsia,but88percentinEurope.Whileinpartsoftheworld,communicablediseasesandreproductiveandchildhealthpresentthemostpressingpublichealthproblems,concernsabouttheprevalenceofage-relatedchronicdegenerativediseasespredominateinothers.Insub-SaharanAfrica,1.2milliondeathsareduetoHIV/AIDS.Althoughthenumbersofsuchdeathsareestimatedtohavepeakedgloballyaround2005(UNAIDS2012),theystillrepresentasubstantialandlong-termburden(UN2011).
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Table6.3.2Distributionofdeaths(%)bycausegroupandworldregion,2010
Communicable Non-communicable
Injuries
Global 24.9 65.5 9.6
Europe 5.1 88.3 6.6
NorthAmerica 5.3 88.0 6.8
High-incomeAsiaandPacific 11.4 81.1 7.5
Oceania 15.1 78.1 6.9
LatinAmericaandCaribbean 14.2 69.0 16.8
NorthAfricaandMiddleEast 16.2 75.2 8.6
Asia(excl.high-incomeandMiddleEast)
23.3 66.2 10.4
sub-SaharanAfrica 66.5 24.9 8.6
Source:datafromGlobalBurdenofDiseaseStudy2010,MortalityResults1970–2010,InstituteforHealthMetricsandEvaluation(IHME),Seattle,Washington,USA,Copyright©2012,availablefromhttp://ghdx.healthdata.org/record/global-burden-disease-study-2010-gbd-2010-mortality-results-1970-2010.
Theprocessthatseparatespopulationswithhighfertility,relativelyhighmortality,youngagestructures,andrapidgrowthfromthosewithlowvitalrates,olderagestructures,andslowornogrowth,isconceptualizedasthedemographictransition.Identifying,andexplaining,thisandassociatedprofoundchangesinhealthhasbeenacentralpreoccupationofmoderndemography(Lee2003).Beforeturningtotheseissues,thebasicmethodsandmaterialsofdemographicanalysismustbeconsideredandtheissueofpopulationdynamics—howpopulationschange—addressed.
Demographicdataandmethodsofanalysis
Intheseventeenthcentury,JohnGraunt,aLondonmerchant,useddatafromtheLondonBillsofMortalitytodeviseanearlylifetable,leadingtohimbeingdubbedthe‘fatherofmoderndemography’.However,whileGraunthadinformationonnumbersofdeaths,helackeddataonthepopulationatriskandcouldnotcomputedeathrates.Essentiallyalldemographicanalysisrequiresdatabothonthepopulation‘stock’andon‘flows’inandout—births,deaths,andmigration.Thetraditionalsourcesofinformationontheformerarepopulationcensusesand,for
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thelatter,vitalregistrationsystems.
Populationcensuses
Thefirst‘modern’censuseswereundertakeninScandinaviaintheeighteenthcentury.CensusesspreadthroughoutEuropeduringthenineteenthcenturyandmostoftherestoftheworldinthetwentieth.Aswellasbasicquestionsaboutage,sex,maritalstatus,andplaceofresidence,dataonothercharacteristicssuchasemployment,educationandhousingareoftencollected.TheUNrecommendsthatcensusesbeconductedatleastdecenniallyinyearsendingin0or1.
Censuseshavemanystrengthsandareoftentheonlysourceofdataforsmallareasorpopulationsubgroups.Althoughprimarilyatoolforcollectingdataonpopulation‘stock’,censuseshavealsobeenusedtofindoutaboutvitalevents.Manycountriesusecensusestoprovidedataonrecentinternalmigration(throughquestionsonplaceofresidence1ormoreyearsearlier)andimmigration(throughquestionsoncountryofbirthand/ordateofentryforthosebornelsewhere).IndirectestimationtechniquesdevelopedbyBrassandothersmeanthatquestionsonnumberofchildrenbornandnumberwhohavedied,onwidowhood,andorphanhoodarewidelyusedtoassessmortalitylevelsandtrendsusingbothcensusesandsurveysincountrieswithdeficientvitalregistrationsystems(Prestonetal.2001).
However,censusesinvolvehugecostsandthechallengeofensuringacceptabledataquality.Approachestoreducingcost(andimprovingquality)includeuseofsamplecensuses,eitherforthecensusasawhole,asinChina,orformoredetailedquestions,asintheUnitedStates.Censustakingrequiresnotonlyareasonableadministrativeinfrastructure,butalsothecooperationofthepopulationtobeenumerated.Somecountrieshavegivenuptakingcensusesbecausethelatterislackingandnowrelyonlarge-scalesurveysor‘virtualcensuses’basedonpopulationregistrationdata.Thetwenty-firstcenturyisseeingmorecountriesadoptingalternativesastheinformationrequiredbygovernmentsbecomesmorecomplexandthedifficultiesofmassdatacollectionescalate.In2010,only11ofthe27currentEuropeanUnionmembersconductedtraditionalcensuses(Valente2010).
Whencensusesaretaken,difficultiesarisingfromerrorsandomissionsarecommon,evenincountrieswithalonghistoryofcensustaking.Young,geographicallymobileadults,recent(especiallyunauthorized)immigrants,membersofminorityethnicgroups,infants,andtheveryoldarethosemostlikelytobeunder-enumerated—someoftheverygroupsthatpolicymakersandhealthprofessionalsmaybemostkeentoknowabout.
Groupssuchasseasonalmigrants,militarypersonnel,peopletemporarilyawayfromhome,andthosewithmorethanoneresidencealsopresentproblems.Notonlyaretheymorelikelytobemissed,butadecisionhastobemadeaboutwhethertheyshouldbeassignedtotheirplaceofusualorlegalresidence(assumingitcanbedetermined),orcountedasbelongingtotheplaceofenumeration.Theformersystemistermeddejure,thelatterdefacto.Theissueofassigningpeopletosomeplaceofusualresidenceisimportantasoftenresourcesareallocatedbasedonpopulationsizeandcharacteristics.Moreover,itisessentialtotryandensurethatdemographiceventsrecordedinonesystem(vitalregistration)areattributedtothepopulationactually‘atrisk’ofexperiencingthem.Inrichercountries,forexample,mostdeathsoccurinhospitalswhichmaydrawpatientsfromawidearea.Ifthesedecedentsarenotassignedtothelocalitywheretheylivedpriortohospitaladmission,areasincludinglarge
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hospitalswillappeartohaveveryhighmortalityrateswhileinothers,recordedmortalitywillbeartificiallylow.
Under-enumerationisusuallyassessedthroughcensusvalidationsurveys(surveysofasampleofcensusaddressesinwhichintensiveeffortsaremadetocontactnon-respondentsandcheckinformationsuppliedbyrespondents)andcomparisonswithpopulationestimatesfromothersources.Beyondensuringnear-completeenumeration,thequalityofthedatacollectedisalsoamajorconcern.
Inmanypopulations,peoplemaynotalwaysknowtheirexactageandsomeapproximationisreportedormadebyanenumerator.‘Heaping’onagesendingin0or5isacommonresult.Heapingcanbedetectedbylookingattheagedistributionandapplyingvarioustestsofconsistencyandsuchdataarenormallyadjustedbeforepublication.Moreseriousproblemsarisewhenreportedageisbasedonothercharacteristics,suchasmaritalstatus,numberofchildrenorgrandparentstatus,asclearlyanyanalysisof,forexample,ageatfirstmarriage,willbebiasedifpeople’sreportoftheirageisinfluencedbytheirmaritalstatus.
Vitalregistration
Dataondemographicevents,aswellasonpopulationcharacteristics,areneeded.Inrichercountriesthesearedrawnfromvitalregistration.CompulsoryregistrationofbirthsanddeathswasestablishedinmostEuropeancountriesduringthenineteenthcentury.InEnglandandWales,forexample,civilregistrationwasintroducedin1837.Subsequentimprovementstothesystemincludedthosefollowingthe1874BirthsandDeathsRegistrationActwhichmadeparentslegallyresponsibleforregisteringbirthsandrequiredattendingphysicianstosupplyinformationoncauseofdeath.Otherrevisionshavesincebeenmade,forexample,theinclusionoffirstmother’sandlaterfather’sageandin2012,recordingofallchildrenpreviouslyborn,ratherthanjustlegitimateones,ontheconfidentialsectionofbirthcertificates.Mosthigh-incomecountrieshavewell-establishedregistrationsystemswithcomplete,orverynearcomplete,coverage.Inpoorerpartsoftheworld,however,vitalregistrationsystemsarefrequentlyseriouslyincompleteornon-existent,althoughthereareexceptionsandsomecountries,includingIndiaandChina,havesampleregistrationsystemsforselectedareas.CurrentlyonlyaboutathirdofdeathsestimatedtooccurgloballyareregisteredandreportedtotheWorldHealthOrganization,althoughifthesampleregistrationsystemsinIndiaandChinaareconsideredassufficientlyrepresentativeoftheirnationalpopulations,thisproportionrisesto72percent(Mathersetal.2005).
Thequalityoftheinformationsuppliedandcodedisofcourseveryimportant.Nooneregisterstheirowndeathandtheinformationobtainedfromproxyinformantsmaybeinaccurate.Differentialreportingofage,occupation,maritalstatus,orothercharacteristicsinthecensusandinothersources,suchasdeathcertificates,presentsafurtherdifficulty.Numerator–denominatordiscrepanciesmayintroduceseriousbiasintotheanalysisofmortalityatadvancedages,orbycharacteristicssuchasoccupationallydefinedsocialclass,maritalstatus,orethnicity(Williamsetal.2006).TheNordicandsomeotherEuropeancountriesavoidtheseproblemsbymaintainingwell-developedregister-basedsystemsthatlinkvitalregistrationdatatopopulation,occupational,andeducationalregisters.Onestudyofmaritalstatusdifferencesinmortalityatolderagesfoundmarkeddifferencesinresultsforcountriesusingtraditionalandregister-basedsystems(Murphyetal.2007).
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Causeofdeath
Deathcertificatesarethemajorsourceofinformationoncauseofdeath.Inrichercountries,causeofdeathisgenerallycertifiedbyaphysicianandcodedaccordingtotheInternationalClassificationofDiseases(ICD)whichoriginatedfromworkundertakenbythenineteenth-centuryBritishmedicalstatistician,WilliamFarr.ThetenthrevisionoftheICDcameintousefrom1994andincludednearlytwiceasmanycodesasICD-9.ICD-11isinpreparationandisexpectedtocomeintousein2015.Nationalpreferences,aswellasICDrevisions,mayinfluenceassignmentofcauseofdeath,asillustratedinanumberofclassicpapersinwhichcasestudiesofdeathsweredistributedtophysiciansindifferentcountries.Growingawarenessofparticularconditionsmayalsoinfluencecodingpractices.IntheUnitedStates,Australia,andelsewhere,forexample,therehavebeenlargeincreasesinmentionsofAlzheimer’sdiseaseandotherdementiasondeathcertificatessince2000.ThesepartlyreflectsomechangesassociatedwiththeintroductionofICD-10butalsoeffectsofincreasedawarenessandsomespecificcampaignstoincreaserecognitionoftheseconditions(Moschettietal.2012).
Olderpeople,nowthevastmajorityofdecedentsinlow-mortalitypopulations,aremorelikelytosuffermultiplepathologiesandthenumberofconditionsrecordedondeathcertificateshasbeenincreasing.Choiceofoneoveranotherasthe‘true’underlyingcauseofdeathisboundtobepartiallyarbitrary.IntheUnitedKingdom,forexample,between1984and1992some25percentfewerdeathswereallocatedtorespiratorydiseasespurelyasaresultofchangesintherulesusedtoselectunderlyingcauseofdeath(GriffithsandBrock2003).Multiplecodingofdeathcertificatesandanalysesbyallmentionsofaconditionmaybemoreinformativebutsuchdataareavailableinonlyafewcountries(Anderson2011).Variationsindeathcertificatecodingreflectingdifferencesinmedicalknowledgeanddiagnosis,intheextenttowhichautopsiesareused,inclassificationsystems,andthequalityofregistrationsystemsareamajorfactorcomplicatinganalysesoftrendsovertimeorbetweencountries.
Deathsassignedtosymptoms,signsandill-definedconditionssuchas‘oldage’or‘senility’orothercauseslackingdiagnosticmeaning,sometimesreferredtoas‘garbagecodes’,presentaparticularproblem.Mathersetal.(2005)inaninvestigationofcoverageandqualityofcauseofdeathcodingin2003,foundthatinsomecountriesover40percentofdeathswereassignedtothese‘causes’.Only23countriesmettheirdefinitionofhighqualityofdatawithcoverageofatleast90percentandfewerthan10percentofdeathsassignedtoill-definedcodes.
Variationsincodingpracticesanduseofill-definedcodescomplicatecomparisonsovertime,aswellasbetweencountries.Preston(1976)arguedthattherewasaninverseassociationbetweentheproportionsofdeathsassignedtocirculatorydiseasesandtoill-definedcauses,andthatpartoftheapparenttwentieth-centuryepidemicinheartdiseasemortalityinrichercountriesmayhavebeenanartefactualconsequenceofimprovementsindeathcertification.AreviewoftheproportionofalldeathsinEnglandandWalesassignedtocirculatorydiseasesandtoill-definedcausesinagegroupsover65from1911–1915to2001–2010isinstructive.Earlyinthetwentiethcentury,largeproportionsofdeathsamongtheveryoldwereassignedtoill-definedcategoriesanddeclinesinthisproportionwereassociatedwithincreasesintheproportionattributedtocirculatorydiseases.Theproportionofill-defineddeathsintheoldestgroupaged80andoverwas,however,slightlyhigherin2001–2010thanintheprecedingperiod,reflectingincreasedassignmentto‘oldage’asacause.Reasonsforthisareunclear,althoughthecessationin1993offurtherenquiryintovaguecausesofdeathmayhavebeenasmallcontributoryfactor.Useofthis‘causeofdeath’islikelytobereversedagaininresponse
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tothe2000–2005publicenquiryintothecaseofHaroldShipman,aBritishfamilydoctorwhoseserialmurderofover250elderlypatientswasnotdetectedformanyyears;anillustrationoftheimportanceofsurveillanceofdeathsforreasonsotherthanepidemiologicalordemographicinvestigation.
Incountrieswhichlackadequatecertificationandregistrationsystems,dataondeathsbycauseareseriouslylimited.Attemptshavebeenmadetodevelopverbalautopsies,protocolsforcollectinginformationfromlayinformantswhichcanbereviewedbyphysiciansandusedtoassigncauseofdeath(Wangetal.2007).Thisapproachhasbeenusefulinanumberofsmallinvestigationsandisbeingemployedonalargerscale,forexample,inIndia(GajalakshmiandPeto2011).However,arecentstudytestedphysician-certifiedverbalautopsiesinsixsitesinfourpoorercountriesagainstgoldstandardassessmentandfoundaconcordanceoflessthan50percent,withsubstantialvariabilitybycauseandphysician(Lozanoetal.2011).Intheabsenceofroutinelyrecordeddata,estimatesmaybeobtainedbymodelling.ThishasbeenusedfortheimportantissueofestablishingthenumberofdeathsfromHIV/AIDS.ThemostambitiousexerciseistheGlobalBurdenofDiseaseprogrammewhichhasusedalargearrayofsourcesandmethods,includingexpertknowledge,vitalregistration,fieldsurveys,surveillance,andpoliceandmortuarydatatoderiveestimatesofcause-specificmortalitybyageandsexfor235separatecausesforeverycountryintheWorld(Wangetal.2012).
Otherdatasources
Manycountrieshavearangeofsurveyswhichprovidemoredetailedinformationon,forexample,health-relatedbehaviour,familybuildingstrategies,reasonsformigration,orinformationonbiomarkersthatwouldbeimpossibletocollectinacensus.Inpoorercountries,whereotherdatasourcesarescarcer,surveysoftenpresentthebestsourceofdataonbasicdemographicparameters.Dataqualityispotentiallybetterinasurveythanacensus,asitismorelikelythatwell-trainedinterviewerscanbeused.TheWorldFertilitySurvey(WFS),aninternationalpopulationresearchprogrammelaunchedin1972todeterminefertilitylevelsthroughouttheworld,anditssuccessor,theDemographicandHealthSurveyProgramme(DHS),havebeenparticularlyvaluableinprovidingdemographicandhealthdataforarangeofpoorercountries.Otherapproachesincludemulti-roundsurveys,inwhichrespondentsareaskedabouteventssincelastcontact,anddual-recordsystemswhichinvolvetwoindependentdatacollectionsystems(oneoftenamulti-roundsurvey),theresultsofwhicharethencombined.Thismethodallowssomeestimationofmissedevents,butisexpensive.Theseapproachesaredescribedinmoredetailinmostdemographictextbooks(Prestonetal.2001;Rowland2003;SiegelandSwanson2004).
Therawmaterialsofdemographyrelatetoindividuals’mostpersonalexperiences—sexualactivity,familyformation,birthcontrol,reproduction,maritalbreakdown,illness,anddeath.Alloftheseoccurinasocialframeworkwhichattachesvaluetosomeofthesebehavioursandstigmatizesothers.Notsurprisingly,respondentsincensusesandsurveysmaybereluctanttodisclosenon-maritalpregnancies,illegalabortions,undocumentedmigration,ordeathsofrelativesfromAIDS.Concealmenthasalsobeenthepolicyofsomenationalgovernmentswhichhavetreateddemographicdataasofficialsecrets.Additionally,theenormouspotentialcomplicationsarisingfrompeople’suncertaintiesaboutageorother‘basic’characteristicsincludingchildrenever-born;uncertainrecollectionsofprioreventsandthevastscopeforadministrativeerrorshavetobeconsidered.Thedemographer’straditionalobsessionwithdata
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qualityishenceunderstandable.Differencesinperceptionsandreportingofhealthstatusarealsoproblematicandhavebedevilledattemptstomakeinternationalcomparisonsofhealthstatusas,evenifquestionsareharmonized,thewayspeoplerespondtothemarenot(Rituetal.2002).
ThestatisticsproducedinseriesliketheUnitedNationsDemographicYearbookshavetheiroriginsinwhatisorhasbeendonebymillionsofpeople,mediatedbywhatissaidabouttheseeventsandexperiences,furtherfilteredbyhowthisisrecorded,processedandanalysed.SomeassessmentofdataqualityisgivenintheUnitedNationsDemographicYearbooks,butsometimesusersmaypayinsufficientattentiontothis.AnumberofotherorganizationsalsoproduceinternationalreferenceworksanddatabasesincludingtheWorldHealthOrganization(WHO),theWorldBank,theOrganisationforEconomicCooperationandDevelopment(OECD),Eurostat,TheUnitedStatesCensusBureauInternationalDataBase(UnitedStatesCensusBureaun.d.),andtheHumanMortalityDatabase.Inmostcases,theseareavailablefreeofchargeonline.
Theanalysisofdemographicdata
Astandardarrayoftechniquesandmeasuresformsthebasisofmuchdemographicanalysis;themostcommonofthesearedescribedbrieflyhere.Furtherdetailissuppliedinanumberoftextbooks(Prestonetal.2001;Rowland2003;SiegelandSwanson2004).Analysisinvolvesnotjusttheapplicationofaparticulartechnique,butalsodecisionsaboutwhatunitsofanalysistouseandhowtogroupthem.Amajordistinctionisbetweenperiodandcohortanalysis.Periodanalysisdealswitheventsofaparticulartimeperiod(e.g.mortalityratesfrom2005to2010)whilecohortanalysesfollowtheexperienceofindividualsthroughtime.Cohortsinthissensearedefinedasgroupsofpeoplewhohaveexperiencedthesamesignificanteventatthesametime.Thusbirthcohortscomprisepeopleborninaparticularyearorgroupofyearsandmarriagecohortsthosemarryingataparticulartime.Cohortandlife-courseapproachestoanalysingmortalityandotherindicatorsofpopulationhealthhaveanintuitiveappealandareincreasinglyused,bothfuellingandfuelledbyagrowingnumberoflongitudinalstudies.Cohortanalysisoftimeseriesdatamaybeusedintheabsenceofspeciallycollectedlongitudinaldata.
Cohortandperiodaretwoofthedimensionswhich‘place’personsintime;thethirdisage.Durationeffects(suchasdurationofmarriage,proximitytodeath,orlengthofexposuretoaparticularpathogen)mayalsobeimportant.Cohorteffectsmaybesubstantialand,unlessallowedfor,maymaskrelationshipsbetweenageandvariousrisks.Differencesinthesmokingbehaviourofcohorts,forexample,haveamajoreffectontherelationshipsbetweenageandsmoking-relateddiseaseobservedatdifferentperiods(Grundy1997).
Decisionsaboutwhethertouseindividuals,families,households,orgeographicareasasunitsofanalysisareoftenconstrainedbydataavailability.Untilrelativelyrecently,mostcensusdatawereonlyavailableasaggregatetabulations,butindividual-levelinformationisincreasinglyavailable.Otherinnovationsincludethedevelopmentofsamplesincludinglinkedcensus,vitalregistration,andinsomecaseshealthservicedatasuchastheLongitudinalStudiesavailableforcountriesoftheUnitedKingdomandanumberofothers(Youngetal.2010).Inthesedatasets,individuals’censusrecordsarelinkedwiththeirvitalregistrationrecordssonumerator–denominatorbiasesin,forexample,theanalysisofmortalityareavoided.InNordiccountries,
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thewholepopulationhasbeenassignedpersonalidentificationnumbersfacilitatinglinkageofinformationfromarangeofregisters.Linkagetouseofhealthandcareservicesisalsoavailableinsomecountries,suchasFinland.
Theseadvanceshavegreatlyextendedthematerialavailableforanalysesofvariationsindemographicbehaviour,andtheirconsequences.Theyhavealsoraisedcomplexsecurityandconfidentialityissuesfuellingdebateoverappropriaterestrictionsonaccesstodata.
Themeasurementoffertility
Fertilitymeansthechildbearingperformanceofawoman,couple,orpopulation.Generallyonlylivebirthsareincluded.Thetermfecundity,bycontrast,isusedtorefertothephysiologicalcapabilityofproducingalive-bornchild.Aroughideaoffertilitymaybegainedfromusingcensusorsurveydatatocalculatechild–womanratios:theratioof0–4-year-oldstowomenaged15–49.However,thesurvivalofinfants(andtheirmothers)andtheagestructureofthefemalepopulationaffectstheseratios,sotheyaregenerallyonlyusedifnootherdataareavailable.
Thesimplestmeasureoffertilitycommonlyusedisthecrudebirthrate—thenumberofbirthsinaparticularyearper1000population.Asthedenominatorofthisincludesthosenot‘atrisk’ofgivingbirth(womenoutsidereproductiveagegroupsandmen),itisreallyaratioratherthanarate.Crudebirthratesareinfluencedbytheagestructureofthepopulation,butlessseriouslysothancrudedeathrates.In2005–2010crudebirthratesrangedfromlessthan10per1000inpartsofEuropetonearly50per1000inthehighestfertilitycountriesofsub-SaharanAfrica.
Slightlymoresophisticatedisthegeneralfertilityratio—birthsper1000womenofreproductiveage(generallydefinedasaged15–49or15–44).Wheredataallow,age-specificfertilityrates(birthsper1000womenofaparticularageoragegroup)arepreferred.Thesearefrequentlysummarizedusingthetotalfertilityrate(TFR).Where,asisusuallythecase,perioddataareusedtocalculatethis,itindicateshowmanychildrenwomeninahypotheticalcohortwouldhaveiftheyexperiencedcurrentage-specificfertilityratesthroughouttheirreproductivelife.ThismeasureissometimesexplicitlydenotedTPFR(totalperiodfertilityrate).Inlow-mortalitypopulations,aTFRof2.1istakentoindicatereplacementlevelfertilityas,underthisregime,acohortofwomenwouldbesucceededbyacohortofdaughtersofthesamesize(aftersomeallowanceformortalityandthefactthat105–106boysarebornforevery100girls).
OnedifficultywiththeTFRisthatitisaffectedbychangesinthe‘tempo’aswellasthe‘quantum’ofchildbearing.Ifwomenstartdelayingtheirfertilitybut‘catchup’later,therewillbeadivergencebetweencohortandperiodmeasures,asthelatterwillbebasedpartlyonthebehaviourofearliercohortswhosetimingofbirthswasdifferent.Similarly,ifwomenhavechildrenearlier,TFRswillrise,evenifeventualfamilysizesremainunchanged.Thismeansthatperiodmeasuresaremuchmorevolatilethancohortones.Forexample,theUSTFR,havingrisenintheearlypartsofthiscentury,fellbymorethan10percent,from2.12to1.89,between2007and2011(Hamiltonetal.2012).Forthesereasons,manystatisticalofficesusecohort,ratherthanperiod,measuresoffertilityasthebasisforprojections.
Moresophisticatedmeasuresoffertilityincludeparityprogressionratios.Theseindicatetheprobabilityofproceedingfromonebirthtoanother(e.g.whatproportionofmotherswithtwo
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childrenprogresstohavingathird).Parityprogressionratiosarenormallycalculatedforcohortswhohavecompleted,ornearlycompleted,theirchildbearingbutitisalsopossibletousedataonbirthsbybirthordertoderiveperiodprogressionratios(BongaartsandFeeney1998).
Inthepast,demographersoftenpreferredtocalculateage-specificmaritalfertilityrates(andTFRsandothermeasures)onthegroundsthattheunmarriedpopulationisnot‘atrisk’(oratreducedrisk)ofchildbearing.Changesinmaritalfertilityindicativeofdeliberateattemptstolimitfamilysizeareregardedasoneofthedefiningfeaturesofthefertility‘transition’(seelater)andsodistinguishingthesefromchangesduetovariationsinthe‘atrisk’(marriedpopulation)hasbeenparticularlyemphasized.However,risesinnon-maritalchildbearing,whichnowaccountforover40percentofbirthsincountriessuchasFrance,theUnitedStates,andUnitedKingdom,meanthatrestrictinganalysestomaritalfertilityisgenerallynolongerappropriate.
Reproductionrates
Intheabsenceofmigrationandwithfixedmortality,populationswillgrowifmothersreplacethemselveswithmorethanone(surviving)daughteranddeclineiftheyhavefewerthanone.Theoretically,itwouldalsobepossibletomeasurethereplacementoffathersbysons,butinpracticethedifficultiesinvolvedinobtainingpaternitydatamakethisinfeasible.Reproductionratesthusrelateonlytofemalefertility—birthsofdaughters.Thegrossreproductionrate(GRR)isderivedinthesamewayastheTFRexceptthatage-specificbirthratesbasedonlyonbirthsofdaughtersareusedinthecalculation.Thenetreproductionrate(NRR)makesanallowanceformortality;specificallythechancethatadaughterwillherselfsurvivetochildbearingage.TheNRRcannotbecalculatedunlessbothage-specificfertilityandmortalitydataareavailable(althoughitcanbeapproximatedusingtheGRRandappropriatelifetablesurvivaldata).Changesineitherfertilityormortality(orboth)willmeanadivergencebetweenperiodmeasures(basedontheexperienceofahypotheticalcohort)andtheexperiencesofrealcohorts.
SummaryinformationonmeasuresoffertilityandreproductionisshowninBoxes6.3.2and6.3.3.
Box6.3.2Fertilitymeasures
DefinitionsFertility:thechildbearingperformanceofindividuals,couples,orpopulations.Fecundity:thephysiologicalcapabilityofproducingalivebirth.Parity:thenumberofchildrenpreviouslybornalive(orsometimesnumberofpreviousconfinements)toawomanorcouple.Nulliparouswomenarethosewhohavebornenochildren.
Measure
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Crudebirthrate:theratioofbirthsinayear(otherspecifiedperiod)totheaveragepopulationinthesameyear/period(mid-yearpopulation),expressedper1000:
Generalfertilityrate:birthstowomenaged15–44/49inayear/periodper1000womenaged15–44/49inthesameperiod:
Age-specificfertilityrate(ASFR):numberofbirthstowomenagedx(orxtox+n)per1000womenagedx(orxtox+n).‘n’referstothelengthofanageinterval.
ASFRsarefrequentlycalculatedfor5-yearagegroupsfrom15–19to40–44or45–49.
Total(period)fertilityrate(TFR/TPFR):thesumoftheage-specificfertilityratesforallreproductiveagegroupsforaparticularperiod(usuallyayear),conventionallyexpressedperwoman.TheTFRindicateshowmanychildrenawomanwouldhaveifthroughoutherreproductivelife,shehadchildrenattheage-specificratesprevalentinthespecifiedyearorperiod.
x=49:
where‘f ’istheage-specificfertilityrateatagex.Ifratesforagegroups,ratherthansingleyears,areusedthenthesumoftheage-specificratesmustbemultipliedbythenumberofsingleagesincludedinthegroup(usuallyfive).
x=45–49:
Parityprogressionratio:theprobabilityofawomenofparityxprogressingtoparityx+1.
Box6.3.3Reproductionrates
Measures
CBR = ×1000.numberofbirths
mid-yearpopulation
GFR = ×1000.numberofbirthstowomenaged15 −44/49
mid-yearpopulationofwomenaged15 −44/49
ASFR = ×1000.birthstowomenagedx
mid-yearpopulationofwomenagedx
TFR = ∑x=15
x=49
fx
x
TFR = 5 ∗ .∑x=15−19
x=45−49
fx
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Grossreproductionrate(GRR):thesumoftheage-specificfemalefertilityrates(birthsofdaughters),forallreproductiveagegroupsforaparticularperiod(usuallyayear)conventionallyexpressedperwoman.TheGRRindicateshowmanydaughtersawomanwouldhaveif,throughoutherreproductivelife,shehadchildrenattheage-specificratesprevalentinthespecifiedyearofperiod.TheGRRcanbecalculatedeitherbysummingfemaleage-specificfertilityrates(relatingtobirthsofdaughtersratherthanallbirths)orusingtheformula:
Theproportionoffemalebirthscanbetakenas0.488(100/205)intheabsenceofmoredetailedinformation.
Netreproductionrate(NRR):theaveragenumberofdaughtersthatwouldbeborne,accordingtospecifiedratesofmortalityandofbearingdaughters,byafemalesubjectthroughlifetotheserates.TheNRRemploysthesamefertilitydataastheGRR,butalsotakesintoaccounttheeffectsofmortality.AnNRRof1.0indicatesthatapopulation’sfertilityandmortalitylevelswouldresultinexactreplacementofmothersbydaughters.
Themeasurementofmortality
Asforfertility,thesimplestmeasureofmortalityisthecrudemortalityrate,deathsper1000population.Thisisstronglyinfluencedbyagestructure.Althoughlifeexpectancyatbirthinthemoredevelopedregionsoftheworldin2005–2010wassome10yearslongerthaninlessdevelopedregions(76yearsand66yearsrespectively),crudedeathrates—deathsper1000populationofallages—werehigherinthemoredevelopedregions(10.0comparedwith8.0inlessdevelopedregions)(UN2011).Age(andsex)specificrates,ormeasuresbasedonthem,arethereforemuchtobepreferredifdataareavailabletocalculatethem.Bothdirectandindirectstandardizationaresometimesusedtomakecomparisonsbetweenpopulationswithdifferentageandsexstructures.Standardizedmortalityratios(SMRs)arecalculatedusingindirectstandardization.Thisinvolvesselectingasetof‘standard’age-specificmortalityrates,forexample,thoseforanationalpopulation,andapplyingthesetothenumbersofpeopleintherelevantagegroupsinthesubpopulationofinterest—forexample,thepopulationofaparticularregion.Thisyieldsan‘expected’numberofdeaths—thenumberofdeathstherewouldbeinthesubpopulationifage-specificdeathrateswerethesameasthoseinthestandardpopulation.TheratioofobservedtoexpecteddeathsgivestheSMR.ThusanSMRof1.24indicatesthatmortalityinthesubpopulationis24percenthigherthaninthestandardpopulation,afterallowingforagedifferences.SMRsareusefulsummarymeasuresofdifferencesinmortality,butgivenoindicationofthelevelofmortality.Indirectstandardization,widelyusedbyWHOandnationalstatisticaloffices,ageandsexspecificratesareappliedtoanexternal‘standard’population,suchastheEuropeanStandardPopulation,toproduceanoverallstandardized(weighted)deathrate.
Age-specificdeathratesarecalculatedusingthenumbersofdeathsatagex(orbetweenagesxandx+n)inaparticularyearasthenumeratorandthemid-yearpopulationofthesameageasthedenominator.Therateisconventionallyexpressedper1000orper100,000population.Themid-yearpopulationisusedasameasureoftheaveragepopulationatriskontheassumptionthatdeathsareevenlydistributedthroughouttheyear.Forsomeagegroups,
GRR = TFR×proportionoffemalebirths.
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notablyinfants,thisassumptionisinvalid.Inlow-mortalitypopulations,deathsinthefirst3daysoflifemayaccountforhalformoreofalldeathsinthefirstyearoflife.Moreover,informationonthesizeofpopulationagedlessthan1normallycomesfrombirthdata(asin9outof10yearsrelevantcensusdatawillnotbeavailable).Forthesereasonslivebirthsinaparticularyearareconventionallyusedasthedenominatoroftheinfantmortalityratewhiledeathstoinfantsagedlessthan1constitutethenumerator.Someinfantsdyinginagivenyearwillhavebeenborninthepreviousyearandsomebornintheyearinquestionwilldiethefollowingyear.Thiscancausedistortionsiftherearelargeannualfluctuationsinnumbersofbirths(orinfantdeaths)andoften3-yearaveragesarepreferred.Deathsatveryoldagesarealsonotevenlydistributedthroughouttheyearandanadjustmentisoftenmadetoallowforthis.
Infantmortalityrates(IMRs)wereveryhighinsomepartsofhistoricalEurope—with300oreven400deathsper1000livebirthsinregionsofRussiaandGermanyattheendofthenineteenthcentury(vandeWalle1986).InEnglandandWalesatthestartofthetwentiethcentury,thereweresome140infantdeathsper1000livebirths.Infantmortalityinhigh-incomecountriesisnowextremelylow—fewerthanfiveinfantdeathsper1000livebirthsinmanyEuropeancountries,Australia,Japan,SouthKorea,HongKong,andSingapore.Therehavealsobeenhugefallsininfantmortalityinmanypoorercountries;in2011ChinaandIndiahadreportedIMRsof13and47respectively(WHO2013).Ratesremainhighinsomeoftheverypoorestcountries—over100deathsper1000livebirths.
Variationsonthisscalehavesubstantialdemographicimpacts.Infantmortalityhasalsoattractedparticularinterestbecauseoflinkswithfertilitybehaviourandasanindicatorofpublichealthconditions.Particularlyinthislattercontext,perinatal,earlyandlateneonatalandpostneonatalmortalityratesareoftendistinguishedwheredataallow(seeBox6.3.4).
Box6.3.4Mortalitymeasures
MeasuresCrudedeathrate:theratioofdeathsinayear(otherspecifiedperiod)toaveragepopulationinthesameyear/period(mid-yearpopulation),expressedper1000:
Age-specificmortalityrate(ASMR):numberofdeathstopersonsagedx(orxtox+n)per1000personsagedx(ortox+n):
Standardizedmortalityratio(SMR):theratioofobservedtoexpecteddeathsinastudypopulation.Expecteddeathsarecalculatedbyapplyingasetofstandardage-specificmortalityratestotheagedistributionofthestudypopulation.Standardizedratiosareonlyusefulforcomparisons.Theyhavenointrinsicmeaning.
CBR = ×1000.numberofdeaths
mid-yearpopulation
ASMR = ×1000.deathstopersonsagedx
mid-yearpopulationofpersonsagedx
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Infantmortalityrate(IMR):
Sometimesdecomposedintoneonatalmortalityrates(deathsofliveborninfantsduringthefirst4weeks)andpost-neonatalmortality(from4to52weeks).
Theperinatalmortalityratemeasureslatefetaldeaths(stillbirths)andearlyneonataldeathsrelativetolivebirths.
Stillbirthsusedtorefertodeathsoffetusesof28ormoreweeks’gestation;however,anearlierthresholdof24weeksisnowmoregenerallyused.
Lifetables
Lifetableanalysisisacoredemographictechniqueandlifetablesprovideoneofthemostpowerfultoolsforanalysingmortalityandotherdemographicprocesses.Lifetablesshowtheprobabilityofdying(andsurviving)betweenspecifiedages.Theyalsoallowthecalculationofvariousotherindicators,includingexpectationoflife.Ifcompletedataonthemortalityofabirthcohortareavailable,thenacohortlifetablemaybeconstructed.However,theuseofcohortlifetablesisobviouslyonlypossibleretrospectively.Morecommonly,periodlifetables,basedonmortalityratesataparticulartime,arecalculated.Theselifetablesshowdeath(andsurvival)probabilitiesforahypotheticalcohortwithanarbitraryradix(numberofbabiesatthebeginning)usuallysetto10,000,100,000orsomeothermultipleof100.
Specificnotation,summarizedinBox6.3.5,isusedinlifetableanalysis.Thebasisofthetableisasetofprobabilitiesofdying— q —whicharecalculatedfromage-specificdeathrates;xherereferstoageatthestartofanintervalwhoselengthisspecifiedbyn.Thus q referstotheprobabilityofsomeonealiveat50dyingbetweenage50andage55.Thecomplementofq —theprobabilityofsurviving—isdenoted p .The(hypothetical)numberofsurvivorsateachageisgivenbyl ;thusl equalstheradix(of100,000)andl thenumberofsurvivorsatage75.Thenumberofpersonyearslivedinaninterval( L )andthetotalnumberofpersonyearslivedafteraparticularage(T )areoftennotshowninpublishedtablesbutarestepsonthewaytothecalculationofe —lifeexpectancyatagex.
Box6.3.5Lifetablemeasuresandnotation
x=ageattainedlastbirthday.
l =theradixofthelifetable(hypotheticalnumberofbabies),usually100,000.
l =numberofsurvivorsatagex,sol isthenumberofpersonsaliveatage65inthehypotheticallifetablepopulation.
×1000.numberofdeathstoinfantsages < 1year
numberoflivebirths
Perinatalmortalityrate
= ×1000.stillbirths+deathsunder1weekstill+livebirths
n x
5 50
n x n x
x o 75
n x
x
x
0
x 65
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q =probabilityofdyingbetweenagexandx+n,so q istheprobabilityofdyingbetweenage1and5forapersonaged1.
p =probabilityofsurvivingbetweenagesxandx+n,so p istheprobabilityofsurvivingfromage65toage85forapersonaged65.
d =numberofdeathsbetweenagexandx+n.
L =numberofpersonyearslivedbetweenxandx+n.
T =totalnumberofpersonyearslivedafteragex.
e =expectationoflifeatagex,soe isexpectationoflifeatbirth.
Thismeasureprovidesanindicatorofmortalitywhichisverylargelyindependentoftheagestructureofthepopulationsinceitdependsonlyonage-specificmortalityrates.Thismakesitmoreusefulthaneitherastandardizedmortalityratio(whichgivesnoindicationoflevel)oracrudedeathrate(whichisstronglyinfluencedbyagestructure).Lifeexpectancyeitheratbirth(e )orfurtherlifeexpectancyataparticularage,say65(e ),iscalculatedbydividingthetotalpersonyearslivedafterage0or65(T orT )bythenumberofsurvivorsaged0(l )or65(l ).
Valuesoflifeexpectancyatbirtharesometimes(mis)interpretedasindicatorsofusualageatdeathinaparticularpopulation.Inverylow-mortalitypopulationswheremostdeathsoccurwithinarelativelysmallrangeofages(seetheexampleofJapaninFig.6.3.2);therewillbeaclosecorrespondencebetweenmedianandmodalagesofdeathandlifeexpectancyatbirth(whichisameanvalue).However,inpopulationssuchasSierraLeonewheresomanydeathsoccurininfancy,therewillbeawidedivergence.Thereisalsosometimesconfusionabouttheinterpretationofvaluesoffurtherlifeexpectancyataparticularage.Thisisderivedfrominformationabouttheprobabilitiesofdeathandsurvivalatsubsequentages,andsoisnotinfluencedbydeathsatearlieragesanditiserroneoustothinkthat,forexample,thefurtherlifeexpectancyofsomeoneaged65willequallifeexpectancyatbirthminus65.Thehigherthemortalityratesatyoungagesthegreaterwillbethisdivergence.In2008,forlifeexample,femalelifeexpectancyatbirthintheUnitedStateswas80.6yearsbutthefurtherlifeexpectancyofwomenaged65was20.0years.Theequivalentfiguresin1900–1901were49yearsand12.0years.
Modellifetables
Patternsofage-specificdeathratesshowsimilaritieswhateverthelevelofmortality.Deathratestendtobehigherininfancythanlaterchildhoodandrisewithagefromaroundtheageofpuberty,althoughintheoldestagegroupsratesofincreasetendtoflattenout.Becauseofthetendencyfordeathratesatoneagetobeassociatedwithdeathratesatotheragesinagivenpopulation,itispossibletoderivehypotheticalschedules,calledmodellifetables,describingvariationsinmortalitybyageandsex,normallyintermsofalimitednumberofparameterswhichallowforparticularfeaturesofthemortalitypatternofthepopulationconsidered.Modellifetablesarederivedfromempiricaldatafromcountrieswheretheseareavailable.Theyareextremelyusefulaidsfortheestimationofmortalitybyageinpopulationswithdefectivedata.
n x 4 1
n x 20 65
n x
n x
x
x 0
0 65
0 65 0
65
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Theyarealsoused(inconjunctionwithfertilitydata)toshowtheoutcomesofparticularfertilityandmortalityregimeson,forexample,populationagestructureandformakingpopulationprojections.Alldemographictextsgivefurtherdetailsoftheirderivationandapplication.
Otherapplicationsoflifetableanalysis
Lifetablesarewidelyusedtoanalyseprobabilitiesassociatedwitheventsotherthandeath,suchasrisksofdivorceorcontraceptiveusefailureanddiscontinuationrates,andinestimatesofdisability-freeorhealthylifeexpectancy.Manychronicconditionsassociatedwithageing,suchasmusculoskeletalandsensoryimpairments,mayhaveseriousimplicationsforhealthstatusbutarenotdirectlylife-threatening.Lifetablemethodsareusedtodecomposetotallifeexpectancyinto‘healthy’and‘unhealthy’or‘disabled’components.Thiscanbedoneusingcross-sectionaldataonmorbidityprevalenceinconjunctionwithmortalitydata,althoughthishassomelimitations.Moresophisticated(anddatademanding)multistateapproacheswhichallowtransitionsbothtoandfromdisabledstateshavealsobeendeveloped(Mantonetal.2006).Despitethesetechnicaladvances,thereisstillcontroversyabouttrendsinindicatorsofthehealthstatusofpopulations,includingdisability.Toalargeextentthisdebatearisesfrommeasurementproblemsandthedifficultiesinvolvedinmakingcomparisonsbetweenhealthindicatorsderivedindifferentways,afurtherreminderoftheimportanceofdataqualityandmeasurement.
Multipledecrementlifetablesallow‘decrements’frommorethanoneevent—forexample,differentcausesofdeath.Causeeliminationlifetablesarealsousedtoidentifythe‘pure’severityofaparticularcauseofdeath.Multistatemodelsallowanalysisofarangeoftransitions,particularlythosewherere-entriesintoaparticularstate,suchasbeingmarriedorlivinginacertainregion,arepossible.Thesemoresophisticatedapplicationsofcourserequiremoredetaileddata.
Themeasurementofmigration
Inmanycountriesmigrationisthepredominantinfluenceonthespatialdistributionofthepopulation.InAsiaandLatinAmericarecentrural-to-urbanmigrationhasresultedinthephenomenalgrowthofcities,oftenlackingtheinfrastructuretomeettheneedsoftheexpandingpopulationforbasicservicessuchassanitationandpower.In2010,52percentoftheworld’spopulationlivedinurbanareascomparedwith29percentin1950(UN2012).
Measuringmigrationrepresentsparticulardifficulties.Theclassicaldefinitionofinternalmigrationisapermanentorsemi-permanentmoveacrossanadministrativeboundary,whichmeansthattheextentofmigrationrecordeddependspartlyonthesizeofadministrativeareas.Forexample,inacountrydividedintomanysmallareas,amoveover5kilometreswillcountasmigration,whichwouldnotbethecaseforcountriesdividedintolargerones.Hence,internationalcomparisonofinternalmigrationratesispotentiallymisleading.Eventhedistinctionbetweeninternationalandinternalmigrationmaybeproblematicifboundariesarecontestedorchanging.Thetemporaldimensiontomigrationpresentsfurtherdifficulties;whatconstitutespermanentorsemi-permanentandhowshouldgroupssuchasseasonalmigrantsbetreated?
Thereasonfordefiningmigrationasamoveoveraboundaryislargelypragmatic.Oftenonly
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movesofthiskindarerecorded;moreoverthisistheinformationrequiredbylocaladministrations.Forresearchpurposes,analysesofallmoves(preferablywithanindicationofdistancemoved)mayoftenbepreferred.Somecountrieshaveregistrationsystemsinwhichchangesofaddressarerecorded.Morecommonly,censusesareusedtofindoutaboutmigration.Questionsonusualaddress1or5yearsagoallowtheproportionofmoversinthepopulationtobemeasured(exceptforthoseagedlessthan1or5).Thesedataalsoallowinflowsandoutflowsbetweenpairsofareastobemeasured.Moves,asopposedtomovers,arenotdirectlymeasuredassomeonemovingseveraltimesinthereferenceperiodcannotbedistinguishedfromsomeonemovingonlyonce.Thoseleavinganaddressandlaterreturningtoitcannotbeidentifiedeither.Thismeansthatthelengthofthereferenceperiodusedisimportant;theproportionofmoversinthe5yearsprecedingacensuswillnotequalfivetimestheproportionmovingin1yearbeforethesamecensus.
Intheabsenceofdirectcensusdata,estimatesofmigrationcanbemadeindirectlyusingthe‘balancingequation’referredtointhefollowingsubsection.Differencesinthesizeofapopulationattwopointsintimenotaccountedforbynaturalchange(i.e.birthsminusdeaths)mustbeduetomigration(ordataerrors).Ifvitalregistrationdataareavailable,thenbothbirthsanddeathscanbetakenintoaccount.Iftheyarelacking,thenthesurvivalofgroupsenumeratedinthefirstofapairofcensusesmustbeestimatedfromalifetableandthenumberofexpectedsurvivorscomparedwiththenumberenumeratedinthesecondcensus(obviouslyageingmustbeallowedfor,sothenumberof20–29-year-oldsinthefirstcensuswillbecomparedwith30–39-year-olds10yearslater).Thesemethodsonlyallowestimationofnetmigration(balancebetweenin-migrationandout-migration).Theirmajorweaknessliesinthefactthattheresidualpopulationbalanceassumedtobeduetomigrationmayinfactreflectdifferencesinthequalityofthetwocensusesconsideredorerrorsintheestimatesofsurvivalused.
Surveydataarealsousedtomeasuremigrationandpotentiallyprovideilluminatinginformationonthereasonsfor,andconsequencesof,migration.However,asmigrationoverlongdistancesisarelativelyrareevent,evenlargegeneralpopulationsamplesmayyieldrelativelyfewmigrants.Asimilarproblembesetssamplesofinternationaltravellers,suchastheUKInternationalPassengerSurvey,designedtoestimateflowsofinternationalmigrantsthroughportorbordersurveys.Touristsandbusinesstravellerscomprisethevastbulkofpeopleenteringorleavingsosurveysareaninefficientwayofidentifyingimmigrantsandemigrants.Unfortunately,otherdataareoftenlackingaslegalandadministrativerecordsystemsarefrequentlyconcernedwithcitizenshipandrightofaboderatherthaninternationalmigrationperse(andvirtuallyneverwithemigration).Estimatesofthesizeofimmigrantpopulationsdependonwhetherthemeasureusedisbasedonplaceofbirthoronnationality—thesizeofthelatterisinfluencedbypoliciesonacquisitionofcitizenship.
Populationdynamics
Anypopulationcomprisesthosewhohavemadeanentryandnotyetexited.Whenwholepopulationsofdefinedgeographicareasareconsidered,theonlymeansofentryarebirthorimmigrationandtheonlymeansofexitdeathoremigration.
Themostbasicmethodofdemographicanalysisisthedecompositionofoverallpopulationchange(P −P )intoitscomponents(B,D,I,E):t 0
− = B−D+I −EPt P0
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whereP =populationatanendofperiod;P =populationatthebeginningofaperiod;andB,D,I,Erepresentrespectivelybirths,deaths,immigrations,andemigrationsduringthesameperiod(B–DisreferredtoasnaturalincreaseandI−Easnetmigration).Populationsubgroupsmaybesimilarlydefinedintermsofentriesandexits.Intheabsenceofmigration,entrytothepopulationaged75–84isthroughageing(passagefrom74to75)exitisthroughfurtherageing(84to85)ordeath.Thissimpleaccountingequationisanimportantone,bothmethodologicallyandasaformalreminderoftheneedtoconsiderpastaswellascurrentevents.
Ofthethreedemographicdeterminantsofpopulationsize,structure,andgrowth,fertilityhashistoricallybeenofmuchgreaterimportancethaneithermortalityormigration.Everybirthrepresentsnotjustanadditiontothecurrentgenerationofchildren,butalsopotentiallyanexponentiallyincreasingaugmentationinthesizeoffuturegenerations.Deathcarriesnosuchpromiseoffuturereturn.Birthsincreasethepopulationonlyatagezero,somakingityounger,whereasdeathsarespreadacrossthewholeagerangeandsohavemuchlessimpactonagestructure.Thethirddeterminant—migration—isgenerallynotofsignificantmagnitudetohaveamajorimpactonmostnationalpopulations,althoughthereareexceptionsespeciallywhennaturalincreaseisclosetozero.IncreasedlevelsofimmigrationtomanyEuropeancountriessincethe1990shavehadquiteaneffectonpopulationsizeandagestructure;thepopulationofSpainforexample,increasedby10.2percentbetween1999and2006andover90percentofthisincreasewasduetomigration(Sobotka2008).
Forsocialandbiologicalreasonsfertility,mortality,andmigrationhaveinteractiveeffects.Decreasesinmortalityamongthosewithreproductivepotential,forexample,influencenotjustthesizeoftheagegroupaffectedatthetime,butalsothesizeofsucceedinggenerations.Declinesinmalemortality,particularlyinpopulationswherelargeagedifferencesbetweenspousesarecommonandremarriageofwidowsisrare,willsimilarlytendtoincreasefertilitybyeffectivelyincreasingtheproportionofwomenofreproductiveagewhoarestillmarried.Conversely,reductionsinfertilityclearlyreducetheriskofmaternalmortalityandmayhavefurtherpositiveeffectsonthesurvivalofmothers,infantsorboth.Ageatmotherhoodalsoinfluencesratesofpopulationgrowth.Theaverageageofmothersatthebirthoftheirdaughtersistermedthemeanlengthofagenerationandisgenerallyaround29years.Ashorterintervalwillmeanmorerapidgenerationalsuccession(andfasterpopulationgrowth).
Migrationaffectsotherdemographicparametersbecausemigrantsdifferfromthegeneralpopulation.Internationalmigrantsaregenerallyyoungandingoodhealthandoftenmovefromrelativelyhigh-tolow-fertilitypopulations.Consequently,immigrantsmayserveto(temporarily)‘rejuvenate’thehostpopulationand,atleastinitially,havehigherfertilityandlowermortality.InEnglandandWales,forexample,24percentofbirthsin2011weretomothersthemselvesbornoutsidetheUnitedKingdom.Despitethedisadvantagestheyoftenface,mortalityofimmigrantgroupsisoftenlowerthanthatofhostpopulationsbecauseofthedifferentialselectionofimmigrants.Thedegreeofselectiontendstovaryaccordingtodifficultiesanddistancetobeovercomeinmakinganinternationalmove.Forallthesereasons,thedemographiccharacteristicsofpopulationsubgroupslargelycomprisingimmigrantsandtheirimmediatedescendantsmayvarysubstantiallyfromthoseofthepopulationasawhole.
Populationprojections
− = B−D+I −EPt P0
t 0
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Populationprojectionsrepresentoneofthemostwidelyusedoutputsofdemographicanalysis.Strictlyspeaking,aprojectionsimplyrepresentstheoutcomeofapplyingvariousassumptionsaboutfuturefertility,mortality,andmigrationandsodiffersfromaforecast,whichimpliesprediction.However,projectionsareoftentreatedasforecastsandthedegreeofuncertaintyinherentinthemisnotalwayssufficientlyrecognized,althoughtheproductionofprobabilisticforecasts,asinthelatestUNprojections,makesthismoreexplicit.Themostcommonmethodofprojectionisthecomponentmethod,basedonthebalancingequation(P =P +B−D+I−E).Assumptionsaremadeaboutthethreecomponentsofchange—births,deaths,andmigration—andappliedtoageandsexgroupswithintheinitialpopulationtogiveaprojectionoffuturesizeandstructure.Toalargeextentassumptionsarebasedonrecenttrendstogetherwithotherinformation,forexample,surveydataonfertilityintentionsor(sometimes)modelsofchangeinparticularcausesofdeath.Forecastingfertilityhasgenerallybeenregardedasthemostproblematicareaofprojectionbutrecentlygreaterattentionhasbeenpaidtotheerrorsthathavebeenmadeinforecastingmortalityindevelopedcountries.Thishaslittleeffectonagegroupsinwhichsurvivalishigh,butcanhavequitesubstantialimpactsonforecastsofthenumberofolderpeople.Migrationmaybeanimportantelementandmaybeunderestimated,ifprojectionsdonottakeintoaccountfeedbackloopswherebymigrantpopulationstendtogeneratefurthermigration(BongaartsandBulatao2000).Internationalmigrationisalsodifficulttoforecastasitisaffectedbyeventsoutsidethecountry,isoftenasensitivepoliticalissueandmaybevolatile.Partlyforthesereasons,immigrationlevelshavebeenconsistentlyunder-projectedinforecastsinmanyEuropeancountries(Aldersetal.2007).
Populationgrowth
Changesinthesizeofapopulationproducedbythesurplus(ordeficit)ofbirthsoverdeathsaretermednaturalincrease(ordecrease).Acommonindicatorofgrowthisthecruderateofnaturalincrease—thedifferencebetweenthecrudebirthrate(annualbirthsper1000population)andthecrudedeathrate(annualdeathsper1000population).Ifnetmigrationiszero,thiswillbethesameasthegrowthrateofthepopulation—theoverallannualchangeinthepopulationdividedbythepopulationsize—(conventionallyexpressedasapercentage).InseveralEuropeancountriesdeathsoutnumberedbirthsintheperiod2005–2010,withthelargestdeficitsinUkraine,Bulgaria,Latvia,Belarus,Hungary,Lithuania,andtheRussianFederation.Insomeothersbirthsstilloutnumberdeathseventhoughfertilityrateshavebeenbelowthelevelrequiredforlong-termreplacementfor40yearsorso.Thisapparentparadoxlargelyreflectsthefactthatthenumberofbirthsisafunctionofthenumberofpotentialmothers,towhichimmigrationmayalsocontribute,aswellasoftheirfertilitypatterns.Iftheformerisincreasingsotoomaythenumbersofbirths,evenifwomenhavefewerchildreneach.
Theyoungagestructuresofmanypopulationsinthedevelopingworldmeanthatthesepopulationshaveahugebuilt-inpotentialforgrowth.Populationmomentumisthemeasurewhichgivestheratiooftheultimatesizeagivenpopulationwouldachievetocurrentpopulationsizeiffertilityweretoimmediatelyfalltoreplacementlevel.EvenallowingfortheeffectofHIV/AIDS-relatedmortality,thepopulationofsub-SaharanAfricaisexpectedtoincreasefrom0.86billionto1.96billionbetween2010and2050(UN2011),aconsequenceofbothpopulationmomentumandhighlevelsoffertility.Insomelow-fertilitycountriestherearenowconcernsabout‘negativemomentum’—theprospectofdeclineinpopulationeveniffertilityratesincreasesomewhatbecauseofsuccessivelysmallercohortsofwomenin
t 0
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childbearingagegroups.
Intrinsicrateofnaturalincrease:stablepopulationtheory
Earlyinthetwentiethcentury,Lotka(1907)demonstratedmathematicallythatapopulationclosedtomigrationandsubjecttounchangingage-specificfertilityandmortalityratesforalongperiodwouldeventuallyhaveafixedagestructure(inwhichtheproportionineachagegroupremainedunchanged)andwouldgrowataconstantrate.Thistypeofpopulationiscalledastablepopulation.Thefixedagestructureofastablepopulationisindependentoftheinitialagestructure—twoverydifferentpopulationssubjecttothesameunchangingratesforalongperiodwouldeventuallyassumethesamestructure.Aparticularcaseofastablepopulationisastationarypopulation—oneinwhichbirthanddeathratesareconstantandinbalanceandsopopulationgrowthiszero.TheL columnofthelifetableisanexampleofastationarypopulation.Thenumberofbirthsisfixed(theradix)andtheagedistributionisalsofixed.Innon-stationarystablepopulations,theagestructureisalsofixedbutthesizeofeveryagegroupisgrowingatthesameconstantrateastheoverallpopulationandthenumberofbirths.Thisiscalledtheintrinsicrateofnaturalincreaseandisafunctionofthenetreproductionrateandthemeanlengthofageneration(approximatedbythemeanageofchildbearing).Non-stationarystablepopulationscanbecalculatedbyadjustingtheL valuesofaparticularlifetabletoallowfortheintrinsicrateofgrowth.Theseareoftenpublishedinconjunctionwithmodellifetablestoshowtheeffectsofparticular(unchanging)fertilityandmortalityregimes.
Althoughstableandstationarypopulationsaretheoreticalconstructs,realpopulationsatvarioustimeshavemetthemodelrequirementscloselyenoughtoallowstablepopulationtheorytobeusedtodevelopmethodsforindirectlyestimatingfertilityandmortalityinpopulationslackingadequatedirectlyderiveddata.Stablepopulationmodelsarealsowidelyusedforinsurance,pension,andpersonnelplanning.OneoftheimportantresultsoftheworkofLotkaandhissuccessorswastoshowtheoreticallytheimportantinfluenceoffertilityonagestructure.
Agestructure
Populationpyramidsgraphicallyillustratethecurrentstructureofpopulationsandinsodoing,alsoprovideinsightsintoboththefutureandthepastofthepopulation.Highfertilitypopulationshaveapyramidshapewitheachsuccessivecohortbeinglargerthanitspredecessor.ThepopulationpyramidforBangladesh(Fig.6.3.3A)showsatypicalpatternforapopulationwithahistoryofhighfertilitybutarecentdownturn.Eachsuccessivecohortislargerthantheprecedingone,withtheexceptionoftheyoungest.‘Old’populations,suchasthatofEnglandandWales(Fig.6.3.3B),aremorerectangularwithagradualtaperingatthetop.Bulgesinpopulationpyramidsduetohighnumbersofbirthshave‘echo’effectswhenmembersoflargecohortsthemselveshavechildren.Thusthebabyboomexperiencedinmanypopulationsinthepost-SecondWorldWarperiod(precisetimingvariedbetweencountries)hadanechoeffectinthe1980s.
x
x
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Fig.6.3.3Distributionofthepopulationof(A)Bangladeshand(B)theUKbyageandsex,2010.
Source:datafromUnitedNations,WorldPopulationProspects:The2010Revision,UnitedNations,NewYork,USA,Copyright©2012,availablefromhttp://esa.un.org/wpp/documentation/pdf/WPP2010_Volume-I_Comprehensive-Tables.pdf
Historically,andapparentlyparadoxically,improvementsinmortalityinthoseEuropeanpopulationswhichnowhavehighproportionsofoldpeopleinfactservedtooffsetthetrendtowardspopulationageing,astheychieflybenefitedtheyoung—andledtoincreasesintheproportionssurvivingtohavechildrenthemselves.However,althoughfertilityhasthegreatestpotentialimpactonagestructureandpopulationgrowth,insomecircumstancesmortality(ormigration)maybecomeamoreimportantinfluence.Manypopulationsinrichercountriesnowhavefertilityatorbelowreplacementlevel,lifeexpectanciesatbirthcloseto80andnearuniversalsurvivaltotheendofthe(female)reproductivespan.Intheseconditions,furtherimprovementsinmortalityhavethegreatestimpactatoldagesandfurtherpopulationageingoccursfromtheapex,ratherthan,orinadditionto,thebaseofthepopulationpyramid.Mortalitychangesarenowthemainmotorofthefurtherageingofanumberofpopulationswithalreadyoldagestructures(PrestonandStokes2012).Populationagestructuresandassociatedratesofgrowthordecline,changesinagestructuressuchaspopulationageing,andthespeedandstageofagestructuralchangeallhaveimportanteconomicandhealthimplicationswhichhaveattractedconsiderabledebateandcontroversy.
Manyeconomistshavepointedoutthatpopulationgrowthhasoftenprovidedaspurtohumaningenuityandeconomicgrowth.Lesspositively,thecountrieswhichnowhavetheyoungestagestructuresandmostrapidratesofpopulationgrowtharealreadysufferingfromlanddegradationandinmanycases,constrainedagriculturalpotential(Alexandratos2005).Largeandgrowingchildpopulationsalsohampereffortsatimprovinghumancapitalthrougheducationorimprovedhealth(Casterline2010).
Reducedfertilityinitiallyproducesa‘demographicdividend’or‘window’whentheratioofchildrentoadultsfallsandthoseinprimeproductiveages,thesurvivorsoflargerbirthcohorts,accountforahigherproportionofthepopulation.Ithasbeenarguedthatthisdividendoflowerchilddependencyandhigherrepresentationofadultsintheprimeworkingagegroupsplayedanimportantpartintherapideconomicdevelopmentofthe‘EastAsianTigers’,likeSouthKoreaandalsoChina(Bloometal.2000).Thenextphase,involvinghighandincreasingrepresentationofolderpeoplemay,ithasbeensuggested,alsobringsomeeconomicbenefitintheformofincreasedsavings(bythelargenumberofolderpeople)andsogreatercapital
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availableforinvestment(MasonandLee2006).However,populationageingismoreoftenperceivedasachallengewithpotentiallynegativeimplicationsforboththeeconomyandforpopulationhealth(OECD1999).
Themajorconcernsarisingfromincreasesintheproportionsofolderpeoplerelatetoeffectsonproductivityandneedforsupportsystemsofvariouskinds,includingpensions,healthcareandlong-termcare.InOECDcountries,healthcareexpenditureistypicallythreetofivetimesashighforthoseaged65andoverasforthoseagedunder65.HoweverthereisconsiderableinternationalvariationintheproportionofGDPdevotedtohealthcarespendingforolderpeoplewhichbearslittleobviousrelationshiptotheproportionofolderpeopleinthepopulationconcerned.
Inpopulationswhichhavemorerecentlymovedtolowfertilityandlowmortalityregimes,thepaceofdemographicchangehasbeenmuchfasterthanoccurredhistoricallyinEurope.TheproportionoftheJapanesepopulationaged65andoverdoubled,from7percenttoover14percent,between1970and1996.InFranceasimilarincreasetook130yearstoachieveandinSweden85years(KinsellaandPhillips2005).Morerecentlyageingpopulationshavethushadamuchshorterperiodinwhichtoadapttonewpublichealthpriorities.Theoriginsoftheseagestructurechangeslieinthedemographictransition.
Thedemographictransition
Towardstheendofthenineteenthcentury(earlierinFrance)birthanddeathratesstartedfallinginanumberofEuropeancountries.Between1871–1875and1911–1915theTFRforEnglandandWales,forexample,droppedfrom4.8to2.8;bytheearly1930sitwasbelowreplacementlevel,adevelopmentwhichwasviewedwithalarmandledtothefirstRoyalCommissiononPopulation.Althoughmodernmethodsofcontraceptionwerelacking,itwasclearthatthishugedropinfertilitywastheresultofthedeliberatelimitationoffamilysize.Halfofcouplesmarriedinthe1870shadsixormorechildrencomparedwith12percentofcouplesmarriedin1911–1915(ColemanandSalt1992).Expectationoflifeatbirth,meanwhile,increasedbysome15yearsbetweentheendofthenineteenthcenturyandtheearly1930s.
Researchersattemptingtorelatesuchshiftsindemographicregimestoeconomicandsocialchanges,originatedthetheoryofthedemographictransition.The‘classical’viewpropoundedbyNotestein(1945)andotherswasthatin‘traditional’societiesfertilityandmortalityarebothhighandroughlyinbalance.Changeisdrivenbyeconomicadvancewhichresultsinlowermortality.Fertilityinitiallyremainshigh,resultinginarapidperiodofpopulationgrowth.Afterthislag,however,fertilityalsofallsinresponsetofallingmortalityandtheerosionof‘traditional’pro-natalistvalues.
Thisclassicalviewhassincebeenconsiderablymodified(Lee2003).CoaleandhiscollaboratorsinanambitiousprojecttotrackthetransitioninhistoricalEuropesuggestedthatnoeconomic‘threshold’forfertilitydeclinecouldbeidentifiedandthatthepatternofdeclineseemedtofollowregionalgroupings,suggestingaculturalratherthanasocioeconomicdimension(CoaleandWatkins1986).Fallsininfantmortality,assumedtobeaparticularlyimportantstimulustofertilitydecline,sometimesfollowedratherthanprecededchangesinfertility.Forexample,Woodsetal.(1989)arguedthatdeclinesinfertilityledtoreductionsininfantmortality,ratherthanviceversainEnglandandWales.Inshort,theroleofmortality
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declineasatriggerandthedominanceofeconomicchangehavebothbeenquestioned.Caldwell(1982)additionallyarguedthatinnon-Europeancountries,itwasnotsomuchsocioeconomicmodernizationbut‘Westernization’involvingincreasedemphasisonthenuclearfamilyandachangeinintergenerationalwealthflows(resultinginthecostsofchildrenoutweighingtheirpotentialbenefits)thatwastheimportanttriggeroffertilitytransition.
ResearchonthehistoricaldemographictransitioninEuropemayseemoflimitedrelevancetocontemporaryproblems.However,thisresearchwasfuelledbypost-warfearsaboutpopulationgrowth.By1950,significantmortalitydeclineshadbeenachievedorinitiatedthroughouttheworld.InChina,forexample,expectationoflifeatbirthincreasedfrom43in1960toover75by2010.Eveninsub-SaharanAfrica,againofnearly10years—from43to52—wasachievedbetween1950and1990,sadlysincereversedalthoughnowstartingtoincreaseagain(WorldBank1993;UN2011).Inthiscontextitseemedimperativetodiscoverthecausesoffertilitydeclineandusethisknowledgetoacceleratefertility‘adjustment’tofallingmortality.Was‘developmentthebestcontraceptive’asconcludedatthestormy1974WorldPopulationConference;couldchangebeachievedthroughintensivefamilyplanningprogrammes,astheexperienceinTaiwanandSouthKoreaseemedtosuggest;orwassomecombinationoftheseandotherfactorsthekeytofertilitytransition?Studiesofsocietiesinwhichthefertilitytransitionhadoccurredseemedtoofferthebestprospectofananswertothesequestions.Whilesimpleanswerstocomplexquestionsarerarelyforthcoming,Coale(1973)identifiedthreefactorswhichheconsideredprerequisitesforfertilitydeclineincontemporarypopulations.Thesewere:thatpotentialparentsmustthinkitacceptabletobalancetheadvantagesanddisadvantagesofanotherchild,thatsomeadvantagemustbegainedfromreducedfertility,andthateffectivetechniquesoffertilitycontrolmustbeavailable.
Trendsinfertilityinthesecondhalfofthelastcenturyandearlypartofthiscenturyhaveshownconsiderabledivergenceandshedsomelightonthesedebates.TFRsarehighestinsub-SaharanAfrica,withUganda,Somalia,Mali,Timor-Leste,andNigerhavingTFRsabove6.3in2005–2010.Inallotherworldregionstherewasclearevidenceofonsetofatransitiontolowerfertilitybythe1970sor1980s;Japanwasthefirstnon-WesterncountrytoexperienceafertilitytransitionstartingaftertheSecondWorldWar.Globally,theestimatedTFRin2005–2010was2.52,with48percentoftheworldpopulationlivingincountrieswithbelow-replacementfertilitylevels(UN2011).Substantialfertilitydeclineshaveoccurredinanumberofcountrieswhichatthattimehadonlyalimitedamountofdevelopment,suchasSriLanka,Thailand,China,andmorerecentlyBangladesh.OneofthelargestandfastestfertilitydeclinestookplaceintheIslamicRepublicofIranwherefertilityfellfrom7.0in1980to1.9in2006(Abbasi-Shavazietal.2009)defyingassumptionsthatsociallyandreligiouslyconservativeMuslimsocietieswithlowratesoffemaleemploymentwouldberesistanttofertilitychange.
Recentinterpretationsoffertilitychangehaverevertedtoconsideringdevelopment—initsbroadestsenseratherthanrestrictedtoconsiderationofaverageincomes—astrongerinfluenceincontemporarypopulationsthanideationalchanges,althoughtherelevanceandroleofculturalandpolicyrelatedfactorsisrecognized(Bryant2007).Commonfactorsidentifiedinpoorruralpopulationswherefertilityhasfallensignificantlyarewell-establishededucationsystems,improvementsinhealthcareandinchildsurvival,someformofextra-familialwelfare,well-organizedlocalgovernment,andanorganizedfamilyplanningprogramme.Potentialbenefitsofinvestingmoreresourcesinfewerchildren,asaconsequenceofincreasingopportunitiesinurbanorindustriallivelihoods,alsoseemtobeimportant
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(McNicoll2006;Bryant2007).Theeducationofwomenhasbeenidentifiedasaparticularlysignificantinfluenceonbothfallingfertilityandimprovedinfantsurvival(Cleland1990;Hobcraft1993)andfemaleeducationandempowermentwererecognizedaskeypolicyobjectivesatthe1994InternationalConferenceonPopulationandDevelopment(UNFPA1995).
Asnotedearlier,fertilityratesremainhighinmuchofsub-SaharanAfrica.ContinuingexpressedpreferencesforlargefamiliesandotherfamilyandsocialcharacteristicsofAfricanpopulationshaveledsometoconcludethatthemodelofchangeseeninAsiaandLatinAmericamaynotbeapplicabletoAfrica,oratleastnottoallregionsofAfrica(Caldwelletal.1992).Thereisneverthelessevidenceofunmetdemandforfamilyplanningandastrongcasecanbemade,onthegroundsofimprovingmaternalandchildhealth,forrenewedinvestmentinfamilyprogrammeswhichinsomeplacesfalteredasthepolicyfocusandfundingshiftedtotheHIV/AIDscrisis(Clelandetal.2006).
Recenttrendshavealsomadeitclearthattheendpointoffertilitytransitionisnotnecessarilyafertilitylevelaroundthe‘replacementrate’.Insomecountries,suchasGuatemala,Peru,Egypt,TurkeyandGhana,fertilitydeclines‘stalled’(sometimestemporarily)atalevelhigherthantwochildrenperwoman(Bongaarts2005).Inotherstheyhavecontinueddownwardspiralling,givingrisetoanewsetofconcernsabout‘lowestlow’fertility.
Lowestlowfertilityandtheseconddemographictransition
Incontrasttothescenarioofhighfertilityandrapidpopulationgrowthinsomeofthepoorestcountries,agrowingnumberofhigh-incomecountriesnowhaveconcernsabouttheimplicationsoflowfertility.Manyofthesecountriesexperiencedapost-SecondWorldWar‘babyboom’duringthe1950sandearly1960sfollowedbya1970s‘babybust’whenfertilitydeclinedtoverylowlevels.InScandinavia,France,theUnitedKingdom,theUnitedStates,andotherEnglish-speakingcountriesfertilityrateshavesincefluctuatedatlevelsbetween1.6andreplacementlevelwithsomethingofanincreaseinthefirstdecadeofthiscentury—apatternalsoseeninmanyotherindustrializedcountries—however,thismayturnouttobeatemporaryphenomenonparticularlygivencurrenteconomicconditions.Thesepopulationshavealsoexperiencedarangeoffamily-relatedbehaviouralchanges,includedmarkedincreasesincohabitation,non-maritalchildbearing,divorce,postponedchildbearing,andincreasedlevelsofchildlessness,describedbysomeasa‘SecondDemographicTransition’(LesthaegheandNeidert2006).Suchbehaviours,whichhavebecomemoreusualamongNorthernEuropeancohortsbornfromaroundthe1950sonwards,aremuchlessprominentinSouthernandEasternEuropeandtherichercountriesofSouthandEastAsia.However,itismainlyintheselattercountriesthat‘lowestlow’fertility—ratesbelow1.3—hasbeenprevalentwith21countriesfromtheseregionshavingTFRsbelow1.3in2003(Goldsteinetal.2009).Therecentincreaseinfertilityindevelopedsocietiesmeantthatonlythreeofthesecountries,SouthKorea,Slovakia,andSingapore,hadaveragevaluesbelow1.3intheperiod2005–2010(UN2011).However,theTFRremainedbelow1.5inalargenumberofothercountriesincludingPoland,Japan,Germany,Italy,Ukraine,Spain,andtheRussianFederationin2005–2010.Animportantdeterminantofverylowfertilityinmanyofthesesocietieshasbeenaprecipitousfallinmarriagerates(BillariandKohler2004).Japan,forexample,hasbeentransformedfromasocietywithnearuniversalmarriageintheearlyandmidtwentiethcenturytooneinwhichafifthofthepopulationwillremainnever-marriedatage45(Retherfordetal.2001).Inthiscase
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theerosionofarrangedmarriagesaftertheSecondWorldWarhasplayedapart;changinggenderrolesmayalsobeimportant.Insomecountrieswithverylowfertility,explicitpro-natalistpolicesareunderdiscussionorhavebeenintroduced,howevertheseremaincontroversialandofuncleareffectiveness.
Theproximatedeterminantsoffertility
Oneofthecontributionsofresearchintothefertilitytransitionhasbeenimprovedunderstandingofbiosocialinfluencesonreproduction.Ahugerangeofsocial,economic,cultural,andpsychologicalfactorsmayinfluencedecisionsaboutfamilybuildingstrategiesandfamilysize.However,thesecanonlyhaveeffectiftheyaretranslatedintopatternsofbehaviourorphysiologicalcharacteristicsthatinfluencetherisksofconceptionordelivery.
Conversely,otherpatternsofbehaviourwithpotentiallyimportantinfluencesonfertilitymaybeadoptedwithlittleornothoughttotheseconsequences.DavisandBlake(1956)distinguishedaseriesof‘intermediatefertilityvariables’:factorsinfluencingexposuretoriskofpregnancy(marriageandcoitalfrequency),riskofpregnancy(suchascontraception),andpregnancyoutcome(spontaneousandinducedabortion).ThemostinfluentialrefinementofthisworkistheBongaartsdecompositionmodel(Bongaarts1978),whichidentifiedfourelementschieflyresponsibleforobservedfertilityvariations:
◆Theproportionofwomenmarried(exposedtorisk).◆Contraceptiveuse.◆Inducedabortion.◆Post-partumnon-susceptibilitytoconception(largelydeterminedbybreastfeedingpractice).
TheTFRisdependentontheinteractiveeffectofthesevariablesandhypotheticalmaximumfertility.Inmodern‘post-transition’populations,fertilitydecisionsarenormallycouple-(orwoman-)basedandareimplementedthroughcontraceptionandabortion.Innon-contraceptingpopulations,biosocialfactors,notablymarriagepatterns,breastfeedingpractices,sexualfrequencyand,insomepopulations,theprevalenceofinfertility,areofmajorimportance.
Thesocialreproductivespan,fromentrytoendofmarriageoranysexualunion,nearlyalwaysstartslater,oftenmuchlater,thanmenarche.Fecundity—thepotentialforbearingchildren—decreasesafterthethirddecade,moresharplyaftertheageof35andinmostnon-contraceptingpopulationstheaverageageatlastbirthisaround40.Socialfactors,aswellasbiologicalones,areimportantinfluences.Sexualactivitymayceasebeforemenopausebecauseofwidowhoodorseparation.InsomeAfricanpopulations,childbearingafterbecomingagrandmotherisdisapprovedof.
Forthosewithintheeffectivereproductivespan—biologicallycapableofchildbearingandinasexualunion—overallfertilityislargelyafunctionoflengthofintervalsbetweenbirths,itselflargelydeterminedbybreastfeedingpatterns.Amongnon-breastfeedingwomen,averagedurationofpost-partumamenorrheaisonly1.5–2months,comparedwith18monthsormorewithprotractedbreastfeeding,particularlyinsomepopulationswheresexualactivityisproscribedforbreastfeedingmothers.
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Longerbirthintervalsandincreasedbreastfeedingalsohavepositiveeffectsoninfantandchildhealth.Overalldeathsbeforetheageof5mightbereducedbyasmuchas30percentinsomecountriesifcloselyspacedbirthsweredelayed(WorldBank1993;Clelandetal.2006).
Theepidemiologicaltransition
Transitionsfromrelativelyhigh-tolow-mortalityregimeshaveinallpopulationsbeenassociatedwithtransformationsintheage,cause,andsexstructureofdeath.Omran(1971)coinedthephrase‘epidemiologicaltransition’todescribethisprocess.Changesintheresponseofsocietiestohealthanddiseaseprocessesalsoneedconsideration.Theterm‘healthtransition’hasbeenproposedasonewhichembracesboththesephenomena.
Substantialfallsindeathratesfrominfectiousandparasiticdiseasesandmaternalmortalityarehallmarksoftheepidemiologicaltransition.InEnglandandWales,overhalfthegaininlifeexpectancyatbirthbetween1871and1911wasduetoreducedinfectiousdiseasemortality.Some20percentofthetotalgainwasduetoreduceddeathratesfromtuberculosis(Caselli1991).Declinesinthesecausesofdeathweregreateramongtheyoung,hencedeathsatolderagesaccountedforalargershareofalldeaths;theepidemiologicaltransitionhasalsobeenconsistentlyassociatedwithlargerfallsinmortalityamongwomenthanmen.Changesintheintra-householdallocationofresources,declinesincausesofdeathprimarilyaffectingwomen(suchasmaternalmortalityandrespiratorytuberculosis),genderdifferencesinhealth-relatedbehaviourandinexposuretooccupationalhazards,andthepossiblygreatersusceptibilityofmentostressesassociatedwithsocioeconomicchanges,mayallbeunderlyingfactors.
Therelativecontributionofvariouseighteenth-andnineteenth-centurydevelopmentsinpromotingthehistoricalepidemiologicaltransitionintheWestremainsamatterofdebate.Improvednutrition,betterhousingandlivingconditions,publicsanitationschemes,andspecificpublichealthinitiatives,suchassmallpoxinoculation,allhavetheiradherents.Intheearlytwentiethcentury,improvedpersonalhygienepracticesandbetterinfantcarewerealsoimportant.Acommonthreadlinkingmostofthesefactorsistheirrelationshiptooverallsocialandeconomicdevelopmentandimprovementsinstandardsofliving.Duringthetwentiethcentury,however,developmentsinmedicaltechnologyandvectorcontrolofferedthepotentialfor‘exogenous’mortalitydeclinelessdependentonaparticularcountry’slevelofincomeanddevelopment.Oneconsequencewasthattherelationshipbetweenpercapitaincomeandlifeexpectancyhasshiftedtotheright(Preston1975,2007).In1901,forexample,lifeexpectancyintheUnitedStateswas49andincomepercapitawasaboutUS$7300(2005purchasingpowerparity).In2009,incomepercapitainChinawasalmostidentical($7400)butlifeexpectancywas73years(http://www.gapminder.org/).
Manypoorcountrieshavebeenabletoachieveremarkablefallsinmortality,especiallychildmortality,throughbehaviouralchange,improvededucationofwomen,andintroductionofrelativelycheaptreatmentsandinterventions,suchasvaccinationandantibiotics(Cutleretal.2006).Between1975–1980and2005–2010lifeexpectancyatbirthincreasedfrom53to68inBangladeshandfrom56to68inIndonesia(UN2011)andtherearenowanumberofpoorormiddle-incomecountrieswithlifeexpectanciesatbirthashighasintheUnitedStates.
Theprocessoftheepidemiologicaltransition(oratleasttheinitialphases)isnowcompleteor
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underwayinmuchoftheworldandnon-communicablecausesofdeathpredominateinallregionsexceptsub-SaharanAfrica(seeTable6.3.2).However,somerecentchangeshavebeenlessbenignandnewchallengesorreversalshaveemerged,notablytheHIV/AIDSepidemicandthehealthconsequencesofthecollapseoftheformerSovietUnion(Olshanskyetal.1997;Murphy2011).Partlybecauseofthesechallenges,therearesignsthatafteraperiodinwhichrisksofmortalityindifferentpartsoftheworldshowedatendencytoconverge(i.e.poorercountriescaughtupwithricherones),morerecentlytherehasbeenatrendtowardsdivergence.Anotherfactorinthismaybethatrecentsuccessesinrichercountriesin,forexample,loweringmortalityfromheartdisease,havepartlybeenachievedthroughtreatmentswhichareharderto‘transfer’topoorcountriesbecauseofinfrastructureandcostlimitations(VallinandMesle2004;Fordetal.2007).
Recentdemographictrendsandpublichealth
Populationsize,growth,andagestructurearealloutcomesofvariationsindemographicbehavioursandallhaveimplicationsforpopulationhealthandwell-being.Populationageingwillalmostcertainlybethepredominantdemographicissueofthetwenty-firstcenturyinnearlyallricherandagrowingnumberofpoorercountries.Thestrongassociationbetweenageandrisksofhealthimpairmentanddisabilityimplygrowingneedsforsupportservices,eveniflevelsofdisabilityfall.Inthosecountrieswhicharegrowingoldbeforetheygrowrich,changesinfamilysupportsystemsforolderpeoplemayposeadditionalchallenges.Arangeofstrategiesforrespondingtothesechallengeshasbeenproposed,includingencouraginglongerworkinglivesthroughreviewofretirementandpensionspolicies,betterorganizationofacuteandlong-termcareservices,andinitiativestopromotehealthyageing(Recheletal.2013).
Changesinmarriageandfamilypatternsalsohaveotherpublichealthimplications.InNorthAmericaandNorthWestEurope(andalsoLatinAmericaandtheCaribbean)highratesofdivorceandnon-maritalchildbearingmeanthatincreasingproportionsofchildrenarespendingatleastpartoftheirchildhoodinlone-parentfamilies.Althoughcausalpathwaysaredifficulttoelucidatebecauseofvariousselectioneffects,thereisevidenceindicatingpoorerhealthamonglonemothersandtheirchildren,andamongunmarried(especiallydivorced)peoplemoregenerally,sothesetrendshavesomenegativeimplications.
Continuingimprovementinchildandadultmortalityisprojectedforthepoorerworld,basedonoptimisticassumptionsaboutthecourseoftheHIV/AIDSepidemic.Inthepoorestcountriestheinteractionofrapidpopulationgrowth,environmentaldegradationandconflictposecontinuinghealthproblemsandthe‘unfinishedagenda’intermsofhealthincludesprovidingaccesstocontraceptionforwomenwhowishtospaceorlimittheirchildren(Clelandetal.2006).Inothermiddle-andlow-incomecountries,patternsoftobaccousearelikelytohaveasubstantialeffectonhealthtrendsincomingdecades(West2006).
Issuessuchasinternationalmigration,economicandcultural‘globalization’,andclimatechangeallhavesubstantialhealthimplicationsfortherestofthetwenty-firstcentury;allinteractwithdemographicpatternsandprocesses.Measuringthesetrendsandassessingtheireffectonhealthanddemandforhealthcarerequiresanunderstandingofpopulationdynamicsandpopulation-basedmeasures,andsuitabledemographicdata.Demographyisthusanessentialcomponentofpublichealth.
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