COHORT STUDYCOHORT STUDY

DR. A.A.TRIVEDIDR. A.A.TRIVEDI(M.D., D.I.H.)(M.D., D.I.H.)

ASSISTANT PROFESSORASSISTANT PROFESSORemail : email : dr_aatrivedi@yahoo.comdr_aatrivedi@yahoo.com

EpidemiologyEpidemiology

Defined by John M. Last in 1988 Defined by John M. Last in 1988 ““Study of Distribution and Study of Distribution and

Determinants of health related state Determinants of health related state or event in a specified population and or event in a specified population and the application of this study to the the application of this study to the control of health problem”.control of health problem”.

We measure – We measure – Disease frequency Disease frequency Diseases distribution Diseases distribution Determinants of disease. Determinants of disease.

TYPES OF EPIDEMIOLOGICAL TYPES OF EPIDEMIOLOGICAL STUDIESSTUDIES

1. OBSERVATIONAL STUDIES

A. DESCRIPTIVE STUDY

DESCRIBE DIESEASE BY

TIME

PLACE

PERSON

B. ANALYTICAL STUDIES

ECOLOGICAL STUDY

CROSS SECTIONAL STUDY

CASE-CONTROL STUDY

COHORT STUDY

2. EXPEREMENTAL STUDIES

RANDOMIZED CONTROLLED TRIAL (RCT)

FIELD TRIAL

COMMUNITY TRIAL

Descriptive EpidemiologyDescriptive Epidemiology

Describe the disease by Describe the disease by Time Time Place Place Person Person

•Cohort study is undertaken to support the Cohort study is undertaken to support the existence of association between existence of association between suspected cause and diseasesuspected cause and disease A major limitation of cross-sectional surveys A major limitation of cross-sectional surveys

and case-control studies is difficulty in and case-control studies is difficulty in determining if determining if exposureexposure or or risk factorrisk factor preceded the preceded the diseasedisease or or outcomeoutcome..

Cohort Study:Cohort Study:

Key Point:Key Point:

Presence or absence of risk factor is Presence or absence of risk factor is determined determined before before outcome occurs.outcome occurs.

WHAT IS COHORTWHAT IS COHORT

Ancient Roman Ancient Roman military unit, A band military unit, A band of warriors. of warriors.

Persons banded Persons banded together. together.

Group of persons Group of persons with a common with a common statistical statistical characteristic. [Latin]characteristic. [Latin]

E.g. age, birth date, E.g. age, birth date,

Cohort studiesCohort studieslongitudinal longitudinal Prospective studiesProspective studiesForward looking study IForward looking study IIncidence studyIncidence study

starts with people free of disease starts with people free of disease assesses exposure at “baseline”assesses exposure at “baseline”assesses disease status at “follow-up”assesses disease status at “follow-up”

INDICATION OF A COHORT INDICATION OF A COHORT STUDYSTUDY

When there is good evidence of When there is good evidence of exposure and disease. exposure and disease.

When exposure is rare but incidence When exposure is rare but incidence of disease is higher among exposedof disease is higher among exposed

When follow-up is easy, cohort is When follow-up is easy, cohort is stablestable

When ample funds are availableWhen ample funds are available

b+d

Frame work of Cohort studiesFrame work of Cohort studies

cc+d

aa+b

Total Yes

Disease Status

Yes

No

Exposure Status

b

d

a+c N

No

Study cohort

Comparison cohort

General consideration while General consideration while selection of cohortsselection of cohorts

Both the cohorts are free of the Both the cohorts are free of the disease.disease.

Both the groups should equally Both the groups should equally susceptible to disease susceptible to disease

Both the groups should be comparable Both the groups should be comparable Diagnostic and eligibility criteria for Diagnostic and eligibility criteria for

the disease should be defined well in the disease should be defined well in advance.advance.

Elements of cohort studyElements of cohort studySelection of study subjectsSelection of study subjectsObtaining data on exposure Obtaining data on exposure Selection of comparison groupSelection of comparison groupFollow upFollow upAnalysisAnalysis

Selection of study subjectsSelection of study subjects

General populationGeneral population Whole population in an areaWhole population in an area A representative sampleA representative sample

Special group of populationSpecial group of population Select group Select group

occupation group / professional group (Dolls study )occupation group / professional group (Dolls study ) Exposure groups Exposure groups

Person having exposure to some physical, chemical Person having exposure to some physical, chemical or biological agentor biological agent

e.g. X-ray exposure to radiologistse.g. X-ray exposure to radiologists

Obtaining data on exposureObtaining data on exposure

Personal interviews / mailed questionnairePersonal interviews / mailed questionnaire Reviews of records Reviews of records

Dose of drug, radiation, type of surgery etcDose of drug, radiation, type of surgery etc Medical examination or special testMedical examination or special test

Blood pressure, serum cholesterolBlood pressure, serum cholesterol Environmental surveyEnvironmental survey

By obtaining the data of exposure we can By obtaining the data of exposure we can classify cohorts asclassify cohorts as Exposed and non exposed and Exposed and non exposed and By degree exposure we can sub classify By degree exposure we can sub classify

cohortscohorts

Selection of comparison groupSelection of comparison group Internal comparisonInternal comparison

Only one cohort involved in studyOnly one cohort involved in study Sub classified and internal comparison doneSub classified and internal comparison done

External comparisonExternal comparison More than one cohort in the study for the More than one cohort in the study for the

purpose of comparisonpurpose of comparison e.g. Cohort of radiologist compared with e.g. Cohort of radiologist compared with

ophthalmologistsophthalmologists Comparison with general population ratesComparison with general population rates

If no comparison group is available we can If no comparison group is available we can compare the rates of study cohort with general compare the rates of study cohort with general population.population.

Cancer rate of uranium miners with cancer in Cancer rate of uranium miners with cancer in general populationgeneral population

Follow-up Follow-up To obtain data about outcome to be To obtain data about outcome to be

determined (morbidity or death)determined (morbidity or death) Mailed questionnaire, telephone calls, personal Mailed questionnaire, telephone calls, personal

interviewsinterviews Periodic medical examinationPeriodic medical examination Reviewing recordsReviewing records Surveillance of death recordsSurveillance of death records Follow up is the most critical part of the studyFollow up is the most critical part of the study

Some loss to follow up is inevitable due to Some loss to follow up is inevitable due to death change of address, migration, death change of address, migration, change of occupation. change of occupation.

Loss to follow-up is one of the draw-back Loss to follow-up is one of the draw-back of the cohort study. of the cohort study.

ANALYSISANALYSIS

Calculation of incidence rates among Calculation of incidence rates among exposed and non exposed groupsexposed and non exposed groups

Estimation of riskEstimation of risk

Incidence rates of outcomeIncidence rates of outcome

N

dc

ba

Yes No

Disease Status

Yes

No

Exposure Status

a+b

c+d

b+d a+c

Total

Study cohort

Comparison cohort

Incidence rateIncidence rate

Incidence among exposed =Incidence among exposed =

aa

a+ba+b Incidence among non-exposed =Incidence among non-exposed =

cc

c+dc+d

Estimation of riskEstimation of risk

Relative RiskRelative Risk

incidence of disease among incidence of disease among exposedexposed

RR = ______________________________RR = ______________________________

Incidence of disease among non-Incidence of disease among non-exposedexposed

a/a+ba/a+b

== __________________

c/c+dc/c+d

Estimation of RiskEstimation of Risk

Attributable RiskAttributable RiskIncidence of disease among exposed Incidence of disease among exposed – incidence of disease among non – incidence of disease among non exposedexposed

AR = _______________________________AR = _______________________________Incidence of disease among exposed Incidence of disease among exposed

a/a+b – c/c+da/a+b – c/c+dAR = _______________AR = _______________

a/a+ba/a+b

Smoking Lung cancer Total

YES NO

YES 70 6930 7000

NO 3 2997 3000

73 9927 10000

Find out RR and AR for above data

Incidence of lung cancer among smokersIncidence of lung cancer among smokers

70/7000 = 10 per 100070/7000 = 10 per 1000 Incidence of lung cancer among non-smokersIncidence of lung cancer among non-smokers

3/3000 = 1 per thousand3/3000 = 1 per thousand

RR = 10 / 1 = 10RR = 10 / 1 = 10

(lung cancer is 10 times more common (lung cancer is 10 times more common among smokers than non smokers)among smokers than non smokers)

AR = 10 – 1 / 10 X 100AR = 10 – 1 / 10 X 100

= 90 % = 90 %

(90% of the cases of lung cancer among (90% of the cases of lung cancer among smokers are attributed to their habit of smokers are attributed to their habit of smoking)smoking)

Types of Cohort StudyTypes of Cohort Study

Prospective cohort studyProspective cohort studyRetrospective (historical) cohort Retrospective (historical) cohort

studystudyCombination of Retrospective and Combination of Retrospective and

Prospective cohort study. Prospective cohort study.

Cohort studiesCohort studies StrengthsStrengths We can find out We can find out

incidence rate and incidence rate and riskrisk

More than one More than one disease related to disease related to single exposure single exposure

can establish cause can establish cause - effect- effect

good when good when exposure is rareexposure is rare

minimizes selection minimizes selection and information and information biasbias

WeaknessesWeaknesses losses to follow-uplosses to follow-up often requires often requires

large samplelarge sample ineffective for rare ineffective for rare

diseasesdiseases long time to long time to

completecomplete expensiveexpensive Ethical issuesEthical issues

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