This document outlines the steps in conducting a cohort study. It defines a cohort as a group of people who share a common characteristic or experience within a defined time period. The key elements of a cohort study discussed are: selection of study subjects, obtaining data on exposure levels, selection of comparison groups, follow up of participants, and analysis of disease outcomes between exposed and unexposed groups to determine the strength of any association. Regular follow up is needed to collect data on disease outcomes.

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STEPS IN COHORT
STUDY

2. CONTENTS
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 Introduction
 What is a cohort?
 Elements of a cohort study
 Selection of study subjects
 Obtaining data on exposure
 Selection of comparison groups
 Follow up
 Analysis
 References

3. INTRODUCTION
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 Cohort study is a type of analytical study
 Done to support or refute the existence of an
association between suspected cause and disease
 Prospective / longitudinal /incidence / forward
looking study
 CAUSE → EFFECT
 Types-Prospective, Retrospective, Combination

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5. 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 COHORTS
 Both should be free of disease, equally susceptible to
the disease, and both groups should be comparable
to each other.

6. FRAMEWORK OF COHORT STUDY
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7. ELEMENTS OF A COHORT
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1.Selection of study subjects
2.Obtaining data on exposure
3.Selection of a comparison group
4.Follow up
5.Analysis

8. 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.

9. 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 members
 Review of records
 Medical examination or special tests
 Environmental surveys

10. 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

11. FOLLOW-UP
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Regular follow up of all participants to obtain data for
assessing the outcome. Few methods are-
 Periodic medical examination
 Review records
 Routine surveillance of death records
 Mailed questionnaires , telephone calls, home visits.

12. 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

13. INTERPRETATION
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14. REFERENCE
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 Park’s Textbook of PREVENTIVE AND SOCIAL
MEDICINE, 23rd edition
 Textbook of community medicine by
AH.SURYAKANTHA, 3rd edition

15. Thank you
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