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
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.
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