This document defines and describes cohort studies. It notes that a cohort study is an analytical study used to provide evidence for or against an association between a suspected cause and disease. It examines populations that are exposed versus unexposed to a potential causative factor and follows them over time to compare disease incidence. The document outlines the key elements of cohort studies, including selection of study subjects, obtaining exposure data, follow up, analysis and comparison groups. It also discusses types of cohort studies, their advantages and disadvantages, examples, and potential biases.

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