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STEPS IN COHORT STUDY

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CONTENTS

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IntroductionWhat is a cohort?Elements of a cohort studySelection of study subjectsObtaining data on exposureSelection of comparison groupsFollow up AnalysisReferences

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INTRODUCTION

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Cohort study is a type of analytical studyDone to support or refute the existence of an

association between suspected cause and disease

Prospective / longitudinal /incidence / forward looking study

CAUSE → EFFECTTypes-Prospective, Retrospective,

Combination

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WHAT IS A COHORT ???

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COHORT-a group of people who share a common characteristic or experience within a defined time period.

STUDY AND CONTROL COHORTSBoth should be free of disease, equally

susceptible to the disease, and both groups should be comparable to each other.

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FRAMEWORK OF COHORT STUDY

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ELEMENTS OF A COHORT

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1.Selection of study subjects2.Obtaining data on exposure3.Selection of a comparison group4.Follow up5.Analysis

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SELECTION OF STUDY SUBJECTS

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• General population- If exposure is frequent, an appropriate sample size(exposed and unexposed) is taken from general population.

• Special groups - - like selected profession, college alumini, antenatal women, etc. -exposure groups-if exposure is rare, select a cohort of persons known to have had exposure.

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OBTAINING DATA ON EXPOSURE

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To know whether they are exposed or not, and also the degree of exposure. Various methods used are:Directly from cohort membersReview of recordsMedical examination or special testsEnvironmental surveys

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SELECTION OF COMPARISON GROUPS

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Internal comparisons- several sub groups based on level of exposure

External comparisons- like smokers and non smokers

Comparison with general population rates-if it is available

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FOLLOW-UP

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Regular follow up of all participants to obtain data for assessing the outcome. Few methods are-Periodic medical examinationReview recordsRoutine surveillance of death recordsMailed questionnaires , telephone calls, home

visits.

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ANALYSIS

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Whether any association exists between the suspected factor and the disease under study

If so, what is the strength of association

COHORT DISEASE +

DISEASE - TOTAL

EXPOSED a b a+b

NOT EXPOSED

c d c+d

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INTERPRETATION

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REFERENCE

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Park’s Textbook of PREVENTIVE AND SOCIAL MEDICINE, 23rd edition

Textbook of community medicine by AH.SURYAKANTHA, 3rd edition

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Thank you

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