1. Presented By
Dr. Fredrick Stephen
P.G in Community Medicine
MGMC&RI, Pondicherry

2. Definition
Cohort study is a type of analytical study which is usually undertaken to
obtain additional evidence to refute or support the existence of an
association between suspected cause and disease.
 Synonyms
Longitudinal study
 Panel study
 Prospective study
 Forward looking study
 Incidence study

3. • What Is Cohort
 Ancient Roman legion, A band of warriors.
 A group of people who share a common
Characteristic or experience within a
defined time period e.g. age , occupation,
pregnancy etc

4. INDICATION OF A COHORT STUDY
 When there is good evidence of exposure and disease.
 When exposure is rare but incidence of disease is higher
among exposed
 When follow-up is easy, cohort is stable
 When ample funds are available
 When attrition is minimal.

5. Design Of A Cohort Study
Population
under
study
Exposed
Develop
outcome
Do not
develop
outcome
Unexposed
Develop
outcome
Do not
develop
outcome

6. Framework of a cohort
Cohort
Disease Total
YES NO
Exposed to putative
aetiologic factor
a b a+b
Not Exposed to
putative aetiologic
factor
c d c+d
Incidence in exposed a/a+b
Incidence in unexposed c/c+d

7. SELECTION OF COHORTS
• Both the cohorts are free of the disease.
• Both the groups should equally be susceptible to the disease
• Both the groups should be comparable
• Diagnostic and eligibility criteria for the disease should be
defined well in advance
• Both the groups are followed up.
• Incidence among exposed is significantly higher than that of
non exposed.

8. Elements Of Cohort Study
 Selection of study subjects
 Obtaining data on exposure
 Selection of comparison group
 Follow up
 Analysis

9. Types of Cohort Study
• Prospective cohort study
• Retrospective (historical) cohort study
• Combination of Retrospective and Prospective cohort
study
• Nested Cohort

10. Cohort study
Advantages Disadvantages
 Can often show temporality of
relationship
 Less bias due to prospective
evaluation of exposures
 Can evaluate multiple diseases
 can establish cause - effect
 good when exposure is rare
 We can find out incidence rate and
Relative risk.
 losses to follow-up
 often requires large sample
 ineffective for rare diseases
 long time to complete
 expensive
 Changes in diagnostic criteria
over time.
 Need motivated cohort of
people who will be repeatedly
evaluated

11. Biases in cohort study
• Analytic bias
• Cross over bias
• Surveillance bias
• Information bias
• Loss to follow up bias
• Interviewer bias
• Non-response bias

12. Examples of Cohort Studies
• Doll and Hills study on smoking and lung
cancer
• Framingham heart study of U.S Public
Health Services
Prospective cohort
study
• Electronic fetal monitoring and neonatal
deaths
• Teenage pregnancy and adverse birth
outcomes
Retrospective cohort
study
• Court Brown & Doll in the effects of
Radiation (for Ankylosing spondylitis)
Combination of
Prospective &
Retrospective study

13. Thank you