cohort study

COHORT STUDYCOHORT STUDY

COHORTCOHORT

– A group of people who share a A group of people who share a common characteristic or experience common characteristic or experience within a defined period of time. e.g. within a defined period of time. e.g. age, occupation, exposure to drug, age, occupation, exposure to drug, exposure to a vaccine, exposed to an exposure to a vaccine, exposed to an infection, insured persons, smokers, infection, insured persons, smokers, obese, pregnant women, babies born obese, pregnant women, babies born on a single day or in a year.on a single day or in a year.

COHORT STUDYCOHORT STUDY

A group of individuals that is A group of individuals that is exposed to a risk factor (study exposed to a risk factor (study factor) is compared with a group factor) is compared with a group of individuals not exposed to a of individuals not exposed to a risk factor (control group).risk factor (control group).

Cohort studyCohort study Cohort study usually proceeds from cause Cohort study usually proceeds from cause

to effect.to effect. Exposure has occurred but the disease has Exposure has occurred but the disease has

not.not. In Cohort Study: In Cohort Study:

– Study group is identified prior to appearance of Study group is identified prior to appearance of disease.disease.

– Control group is identified prior to the Control group is identified prior to the appearance of disease.appearance of disease.

Study groups are observed over a period Study groups are observed over a period of time to determine the frequency of of time to determine the frequency of disease among them.disease among them.

Cohort studyCohort study

Risk Risk factorfactor

DiseaseDisease

yesyesDiseaseDisease

nonototaltotal

PositivePositive aa bb a+ba+b

Negative Negative cc dd c+dc+d


Cohort must be free from disease under Cohort must be free from disease under studystudy

Both study and control groups must be Both study and control groups must be equally susceptible to disease under studyequally susceptible to disease under study

Both the groups should be comparable in Both the groups should be comparable in respect to all possible variablesrespect to all possible variables

Diagnostic and eligibility criteria of the Diagnostic and eligibility criteria of the disease must be defined beforehand.disease must be defined beforehand.

Groups must be followed under same Groups must be followed under same identical conditions over a period of time identical conditions over a period of time to determine outcome of exposure.to determine outcome of exposure.



DiseaseDisease


nonototaltotal

PositivePositive aa bb a+ba+b

Negative Negative cc dd c+dc+d


After end of follow upAfter end of follow up– We can calculateWe can calculate

Incidence rate among exposedIncidence rate among exposed



Incidence rate among exposedIncidence rate among exposed= a / a + c= a / a + c




Incidence rate among non exposedIncidence rate among non exposed

=c / c + d=c / c + d




Incidence rate among non exposedIncidence rate among non exposed

=c / c + d=c / c + d

Cohort study is most reliable mean of Cohort study is most reliable mean of showing an association between a showing an association between a suspected risk factor and subsequent suspected risk factor and subsequent diseasedisease..


Elements of cohort studyElements of cohort study– Selection of study subjectSelection of study subject– Obtaining data on exposureObtaining data on exposure– Selection of comparison groupSelection of comparison group– Follow upFollow up– AnalysisAnalysis

STEPS IN CCSSTEPS IN CCS

Selection of cases and controlsSelection of cases and controls MatchingMatching Measurement of exposure andMeasurement of exposure and Analysis and interpretationAnalysis and interpretation

1.Selection of study subjects1.Selection of study subjects

General populationGeneral populationSpecial groupSpecial group

Select groups------homogeneous groupSelect groups------homogeneous groupExposure group---experience of Exposure group---experience of

exposureexposurePhysicalPhysicalChemicalChemicalDisease agentDisease agent

2. Obtaining Data On Exposure2. Obtaining Data On Exposure

Cohort membersCohort membersPersonal interviewsPersonal interviewsMailed questionnaireMailed questionnaire

Review of recordsReview of records Medical examination or special testsMedical examination or special tests Environmental surveysEnvironmental surveys Information required for:Information required for:

– Age, sex, gender & demographic Age, sex, gender & demographic variables etc.variables etc.

3. Selection of comparison group3. Selection of comparison group

Internal comparisonsInternal comparisons

External comparisonsExternal comparisons

Comparison with general Comparison with general population ratespopulation rates

4. Follow up4. Follow up

Regular follow up of all participantsRegular follow up of all participants Methods should be devised to measure Methods should be devised to measure

outcomeoutcome Procedures required are;Procedures required are;

Periodic medical examination of each memberPeriodic medical examination of each memberReviewing of physician, medical & hospital Reviewing of physician, medical & hospital

recordrecordRoutine surveillance of death recordsRoutine surveillance of death recordsMailed questionnaire, telephone calls, periodic Mailed questionnaire, telephone calls, periodic

home visits etc.home visits etc.

5.Analysis5.Analysis

Incidence rates of outcome Incidence rates of outcome – Among exposedAmong exposed– Among non-exposedAmong non-exposed

Estimation of riskEstimation of risk– Relative riskRelative risk– Attributable riskAttributable risk

CIGARETTE SMOKING AND LUNG CIGARETTE SMOKING AND LUNG CANCERCANCER

CigaretteCigarette

smokingsmokingDevelopeDevelope

d Lung d Lung cancercancer

Did not Did not develop develop

Lung Lung cancercancer

TotalTotal

YesYes

NoNo

7070

(a)(a)

33

( c)( c)

69306930

(b)(b)

29972997

(d)(d)

70007000

(a+b)(a+b)

30003000

(c+d)(c+d)

1.Incidence rates1.Incidence rates

Among exposed = a/ a+b=70/7000Among exposed = a/ a+b=70/7000

= 10 per 1000= 10 per 1000

1.Incidence rates1.Incidence rates

Among exposed = a/ a + b = 70 / 7000Among exposed = a/ a + b = 70 / 7000

= 10 per 1000= 10 per 1000

Among non exposed= c/ c + d =3 / Among non exposed= c/ c + d =3 / 30003000

= 1 per 1000= 1 per 1000

P<0.001P<0.001

Cohort study (CHD)Cohort study (CHD)


DiseaseDisease


nonototaltotal

BankersBankers aa

6060bb

4040a+ba+b

100100

Non Non bankers bankers

cc

3030dd

7070c+dc+d

100100

Cohort study (HTN)Cohort study (HTN)


DiseaseDisease


nonoTotalTotal

obeseobese aa

100100bb

900900a+ba+b

10001000

Non Non obese obese

cc

3333dd

967967c+dc+d

10001000

2.ESTIMATION OF RISK2.ESTIMATION OF RISK

Relative riskRelative riskincidence among exposedincidence among exposed

RR= _________________________ RR= _________________________ incidence among non exposedincidence among non exposed

RR=10/1=10(sRR=10/1=10(smokers are 10 times greater risk of mokers are 10 times greater risk of developing Ca Lung)developing Ca Lung)

It is a direct measure of strength of It is a direct measure of strength of association between suspected cause and association between suspected cause and effect.effect.

RR=1RR=1 no associationno associationRR=>1RR=>1 positive association b/w expos & positive association b/w expos &

dis.dis.

ATTRIBUTABLE RISKATTRIBUTABLE RISK Is the difference in incidence rates of Is the difference in incidence rates of

disease between an exposed group disease between an exposed group and non exposed group. It is and non exposed group. It is expressed as percent.expressed as percent.

incidence of disease rate incidence of disease rate among AR=exposed—incidence among AR=exposed—incidence of disease rate of disease rate among non among non exposedexposed

incidence rate among exposedincidence rate among exposed

AR=10-1/10X100=90% (90% of lung AR=10-1/10X100=90% (90% of lung cancer was due to lung cancer).cancer was due to lung cancer).

Advantages of Cohort studyAdvantages of Cohort study

Incidence can be calculatedIncidence can be calculated Several possible outcomes related to Several possible outcomes related to

exposure can be studiedexposure can be studied Direct estimate of relative riskDirect estimate of relative risk Dose response can be calculatedDose response can be calculated Minimized biasMinimized bias

Disadvantages of cohort studyDisadvantages of cohort study

Large no. of people are involvedLarge no. of people are involved Takes long time to complete the studyTakes long time to complete the study Administrative problems, loss of staffAdministrative problems, loss of staff Loss of original cohortLoss of original cohort Selection of comparison groupsSelection of comparison groups Change in standard methodsChange in standard methods ExpensiveExpensive Change in people behaviourChange in people behaviour

Differences between case control and cohort studiesDifferences between case control and cohort studies

Case control studyCase control study Cohort studyCohort study1. Proceeds from “effect to cause1. Proceeds from “effect to cause Proceeds from “cause to effect”Proceeds from “cause to effect”

2. Starts with the disease2. Starts with the disease Starts with people exposed to risk factor Starts with people exposed to risk factor or suspected cause.or suspected cause.

3. Tests whether the suspected cause 3. Tests whether the suspected cause occurs more frequently in those with the occurs more frequently in those with the disease than among those without the disease than among those without the disease.disease.

Tests whether disease occurs more Tests whether disease occurs more frequently in exposed, than in those not frequently in exposed, than in those not similarly exposed.similarly exposed.

4. Usually the first approach to the 4. Usually the first approach to the testing of a hypotheses, but also useful testing of a hypotheses, but also useful for exploratory studiesfor exploratory studies

Reserved for testing or precisely Reserved for testing or precisely formulated hypothesisformulated hypothesis

5. Involves fewer number of subjects5. Involves fewer number of subjects Involves larger number of subjectsInvolves larger number of subjects

6. Yields relatively quick results6. Yields relatively quick results Long follow-up period Long follow-up period

7. Suitable for the study of rare diseases7. Suitable for the study of rare diseases Inappropriate when the disease or Inappropriate when the disease or exposure under investigation is rare.exposure under investigation is rare.

8. Generally yields only estimate of RR 8. Generally yields only estimate of RR (odds ratio)(odds ratio)

Yields incidence rates. RR as well as AR.Yields incidence rates. RR as well as AR.

9. Cannot yield information about 9. Cannot yield information about diseases other than that selected for diseases other than that selected for studystudy

Can yield information about more than Can yield information about more than one disease outcome.one disease outcome.

10. Relatively inexpensive10. Relatively inexpensive ExpensiveExpensive

cohort study

Documents

study groups

cohort studycohort study

calculateincidence rate

studyboth study

risk factor study factor

appearance of disease

cincidence rate

subsequent disease