descriptive epidemiology & study design - … key concepts “descriptive epidemiology” is the...
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Descriptive Epidemiology & Descriptive Epidemiology &
Study designStudy design
Potjaman Potjaman SiriarayaponSiriarayaponBureau of EpidemiologyBureau of Epidemiology
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Outline
General concept of descriptive epidemiology
Study design in epidemiologyDescriptiveAnalytic
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KEY CONCEPTS
““Descriptive epidemiologyDescriptive epidemiology”” is the first step in epidemiological study. Careful observation of available information alone have led many successful preventive measures in the past.Describing health or disease occurrence according to place, person and time can lead to very meaningful hypothesis formulations.
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TIME
Disease rates change over time. Some of these change occur regularly and can be predicted.
By examining events that precede a disease rate increase or decrease, we may identify causes and appropriate actions to control or prevent further occurrence of the disease.
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0
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
6 0 0
1 9 6 0
1 9 6 5
1 9 7 0
1 9 7 5
1 9 8 0
1 9 8 5
1 9 9 0
Year
Reported cases (per 1,000)
Vaccine Licensed
Measles by year of report, United States, 1960-1989
Secular (longSecular (long--term) trends:term) trends: to predict or evaluate control program
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Seasonality:Seasonality: to see seasonal pattern to see seasonal pattern over yearsover years
0
2
4
6
8
1 0F e
b
Ap r
Jun
Aug O
c t
De c
9 79 89 9
Cases of eosinophilic meningitis, Nan province,Thailand, 1997-1999
Cases
Onset by Month
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Day of week and time of day:Day of week and time of day: especially especially important for condition that related to important for condition that related to
occupational or environmental conditionoccupational or environmental condition
0
5
1 0
1 5
2 0
2 5
3 0
8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8
Deaths
Time of Day
Fatalities associated with farm tractor injuriesby time of day, Georgia, 1971-1981
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Epidemic period:Epidemic period: to show the time course to show the time course of disease outbreakof disease outbreak
0
10
20
30
40
19 21 23 25 29 1 3 5 7 9 11 13 15 17 19 21
FloodCases
Date of onsetNumber of leptospirosis patientsby date of onset, Nov 25-Dec 21, 2000
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PLACE
We describe a health event by place to gain insight into the geographical extent of the problem.
We may use place of residence, birthplace, place of employment, school district, hospital unit, etc., depending on which may be related to the occurrence of the health event.
MyanmarUmphangdistrict
Nov.-Dec.
Jan-Feb.
Mar.
villageRefugeecamp
Meningococcal cases,Meningococcal cases,TakTak province, province, 20022002--2003 2003
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Thai-Myanmar border area in Umphangdistrict
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In the refugee camp
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House of Karen people outside the camp
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PERSON
There are several person categories available:
inherent characteristics: age, sex, race
acquired characteristics: marital status, immune
activities: occupation, use of medication/tobacco/drugs
condition under which their live: socioeconomic status, access to medical care
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Sex specific attack rates of reported Sex specific attack rates of reported leptospirosisleptospirosis casescases,, SongklaSongkla,, NovNov 2525--DecDec15, 200015, 2000
5 9 . 5
2 0
0
2 0
4 0
6 0
8 0
M a l e F e m a l e
S e x
C a s e s / 1 0 0 , 0 0 0
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Age Age specificspecific attack rates of attack rates of reportedreportedleptospirosisleptospirosis casescases,, SongklaSongkla,, NovNov 2525--DecDec 15, 15, 20002000
21.6
33.9 37.3
51.140.7
58.3
16.7
0
20
40
60
80
0-9 10-19 20-29 30-39 40-49 50-59 60+
Age group
Cases /100,000
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DEFINITION OF EPIDEMIOLOGYDEFINITION OF EPIDEMIOLOGY
"Epidemiolgy is the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems"
(John M. Last, 1988)
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Study designStudy design
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Why Why ““type of studytype of study””??
•Design more appropriate studies to get right answers to the specific questions
• Getting more optional study design to the questions
• Shape up proposals for getting more valid results
TYPES OF OBSERVATIONAL STUDIES
DescriptiveStudies
Little is knownabout theoccurrence, ordeterminants of thedisease
- Identify cases, estimate diseasefrequency, examine time trend
- Justify additional (analytic)studies to test hypotheses
AnalyticStudies
Enough is knownabout the ddisease,specific hypothesescan be tested
- Test specific etiologic hypotheses
- Justify additional studies
- Suggest potential for diseaseprevention
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STUDY DESIGN OF DESCRIPTIVE
STUDY
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STUDY DESIGN OF DESCRIPTIVE STUDYSTUDY DESIGN OF DESCRIPTIVE STUDY
Individual levelCase reports or case seriesCross-sectional surveys of individual
Population levelEcological study (Correlational study)
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CASE REPORTS AND CASE SERIESCASE REPORTS AND CASE SERIES
Case reports Case reports :: describe experience of a single patients. Case reports document unusual medical occurrence and can represent the first clues in the identification of new diseases.
Case series Case series :: collections of individual case reports. Investigation of the activities of the individual in case reports can lead to formulation of a hypothesis.
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Streptococcus suis
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CASE REPORTS AND CASE SERIES
AdvantageAdvantagediscover new diseases
bring background information to form hypothesis about risk factors
DisadvantageDisadvantagemight base on only one casedo not have a comparison group
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CROSSCROSS--SECTIONALSECTIONAL STUDYSTUDY
Total population
Cases
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Character of cross-sectional study
Collect data for all population; case & healthy people
Information obtain is the number of total cases among total population at that study period: prevalence
Can be either descriptive or analytic, depend on design
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CROSS-SECTIONAL STUDY
Descriptive Descriptive Collected number of cases and number of total population
Can assess only prevalence of disease or other health events, also called prevalence study
AnalyticAnalyticExposure anddisease status areassessedsimultaneously
Can determine association between exposure and disease
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Vaccine coverage among Thai children
Proportion of children received vaccine
Education level of mother and vaccine status in children
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CrossCross--sectional study sectional study
Defined Population
ExposedExposed
Have disease
Not exposed:
Have disease
NotExposed:
Do nothave disease
ExposedExposed
Do nothave disease
Gather Data on Exposure and Disease
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Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease
Job A Job A ((hazardoushazardous))
100100WorkersWorkers
80 well
20 ill
Job B (non-hazardous)
100Workers
95 well
5 ill
80 well
1010 illill
10 ill
95 well
1515 illill
10 change jobsdue to illness
Point X
Prevalence of job A = 20 % ( 20/100 )Prevalence of job B = 5 % ( 5/100 )Prevalence ratioPrevalence ratio == 44
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Hypothetical illustration of the interrelationship between an occupational exposure and prevalence of disease
Job A Job A ((hazardoushazardous))
100100WorkersWorkers
80 well
20 ill
Job B (non-hazardous)
100Workers
95 well
5 ill
80 well
1010 illill
10 ill
95 well
1515 illill
10 change jobsdue to illness
Point YPrevalence of job A = 11 % ( 10/90 )Prevalence of job B = 14 % ( 15/110 )Prevalence ratioPrevalence ratio == 0.80.8
AdvantageEasy and less time consumedEstimate magnitude of problem especially for rare diseaseFirst step for exploring and forming hypothesisAble to use data for further case-control or
cohort during analytic study
DisadvantageDifficult to confirm that risk factor come before disease (temporal relationship)Length biased sampling: Disease that has long duration will over-represent the magnitude of illness while short duration will under-represent illness.
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STUDY DESIGN OF DESCRIPTIVE STUDYSTUDY DESIGN OF DESCRIPTIVE STUDY
Individual levelCase reports or case seriesCross-sectional surveys of individual
Population levelEcological study (Correlational study)
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ECOLOGICAL STUDYECOLOGICAL STUDY
These studies are an extension of the use of routine data.
Essentially the average exposure of the population is plotted against the rate of the outcome for that population
This is done for several populations and the data are then examined for evidence of an association between exposure and outcome.
Group of Proportion of Suicide rate Prussian Province Protestant (per 100,000 pop.)
A 9.56 0.30
B 16.36 0.45
C 22.00 0.79
D 26.46 0.95
Study to find relationship between suicide rate and proportion of Protestant
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0
0.2
0.4
0.6
0.8
1
0 10 20 30
A
B
C
D
Prov. Proportion of SuicideProtestant rate
A 9.56 0.30
B 16.36 0.45
C 22.00 0.79
D 26.46 0.95
Suicide rate (per 10 5/year)
Proportion of Protestant (%)
Correlation between proportion of protestant and suicidal rate in 4 groups of Prussian provinces
Inci
denc
e
Average temperature15.3 24.3
.33
14.34
Correlation of JE incidence and average temperature in Dali prefecture,
Yunnan province, China, 2001
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Rationale for ecological studyRationale for ecological study
Low cost and convenience: make use of existing data
Measurement limitations of individual-level studies: environmental study
Interest in ecological effect: social norm,population intervention
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ECOLOGICAL STUDYECOLOGICAL STUDY
AdvantageAdvantage:: cheap, quick and convenient since it usually come from existing data
DisadvantageDisadvantage::
inability to link exposure with disease in individual (ecological fallacy)limit to control effect of other factors
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In summary
• Ecological study is a convenient, cheap and simple study.
• Unit of study is the aggregate data not individual level.
• It is usually be conducted as the first step study for research.
• The result is difficult to interpret because of confounding and bias.
Ecological or correlation study Ecological or correlation study
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Study design
Observational Experimental
Descriptive
Case and serial case
Cross-sectional
Ecological
Analytic
Case-control
Cohort
Quasi-experimental
Experimental
Cross-sectional
Ecological
Summary of the study designSummary of the study design
Analytic study
Disease among exposed?
Disease among non-exposed?
UsuallyUsually prospectiveprospective
Populationat risk
Cohort studyCohort study
Exposed
Not Exposed
and
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Distribution of illness according Distribution of illness according to exposure in a cohort studyto exposure in a cohort study
Exposed
Not exposed
Ill Not ill
a b
c d
a+b
c+d
RiskRisk
a+b
c+d
a
c
Relative risk = Risk Relative risk = Risk exposedexposed / Risk / Risk not exposednot exposed
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Key featuresShould have follow up period (even in retro-cohort)
Have confine population
Comparison group should be as similar as possible
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Cohort study Cohort study
AdvantageAdvantage
Determine multiple effect of single exposureAble to estimate incidenceMinimize bias for prospective study
DisadvantageDisadvantageExpensive and time consumedValidity of result depend on follow up
Disease(Case)
Exposure
?
No disease(Control)
?
CaseCase--control studycontrol study
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Distribution of cases and controls according Distribution of cases and controls according to exposure in a caseto exposure in a case--control studycontrol study
Exposed
Not exposed
Total
Cases
a
c
a+c
Odds of exposure a/cOdds of exposure a/c
Controls
b
d
b+d
b/db/d
Odds ratio = OddsE (cases) / OddsE (controls)
Odds
Team A plays 10 games:8 wins2 defeats
Risk (probability) of losing = 2/10Odds of losing = 2/10 = 2/8
8/10
Probability of eventProbability of non-event
Key features
Study only some part of the population, especially among non-case
Case and control should come from the same source population, to allow possibility that control could receive the exposure
Case and control are not necessary to be the same
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Advantage Advantage Cheap, quick and efficient for rare Cheap, quick and efficient for rare diseasediseasePotential role for testing multiple risk Potential role for testing multiple risk factorsfactors
DisadvantageDisadvantageSusceptible to biasSusceptible to biasInefficient for rare exposureInefficient for rare exposureSometime difficult to establish Sometime difficult to establish temporal relationship between exposure temporal relationship between exposure and diseaseand disease
CaseCase--control study control study
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Advantage:Advantage:•• Feasible and practical especially for Feasible and practical especially for ethical concernethical concern•• Study population is more represent of Study population is more represent of the target population the target population
Limitation: Limitation: •• Difficult to replicate Difficult to replicate •• Less control of extraneous factors that Less control of extraneous factors that will lead to distortion of the resultwill lead to distortion of the result•• Less secure to make generalization Less secure to make generalization
OBSERVATIONAL STUDYOBSERVATIONAL STUDY
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Experimental orExperimental orintervention studyintervention study
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Experimental study Experimental study
• Put afford to control the situation of study factors and observe effect of them
• Control extraneous factors by holding those factors fix or “randomization” to make two groups have the same distribution of extraneous factors.
• One group of subject is given experimental treatment and another group (control) is given either none or less preferable. After a period effect of exposure is measured and compared between two groups.
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• RandomizationRandomization is used to allocate subject: subject get treatment by chance
• One important method to make the result more valid is called ““double double blindingblinding””:: neither investigator nor subjects know the treatment to which they have been allocated
Experimental study Experimental study
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Limitation:Limitation:• Effect is observed in artificial setting because the controlling extraneous factors
• Randomization may not be ethical
• Apart from pharmaceutical intervention, other intervention is difficult to do double blinding
Experimental studyExperimental study
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Study design
Observational Experimental
Descriptive
Case and serial case
Cross-sectional
Ecological
Analytic
Case-control
Cohort
Quasi-experimental
Experimental
Cross-sectional
Ecological
Summary of the study designSummary of the study design
Thank you, Khob Khun ka
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ECOLOGICAL STUDY
Exploratory studyExploratory study:: if there is no specific exposure of interest or the exposure of potential interest is not measureAnalytic studyAnalytic study:: if the primary exposure variable is measured and included in the analysis
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Main feature of each type of epidemiologic research
Experimental : Experimental : artificial artificial manipulation of study factormanipulation of study factorwith with randomizationrandomization
QuasiQuasi--experimental study : experimental study : artificial manipulation of the study factor artificial manipulation of the study factor without randomizationwithout randomization
Observational Observational ::no no artificial manipulationartificial manipulation of the study of the study factor factor