PREVENTIVE MEDICINE

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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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