Presented By

Dr. Fredrick Stephen

P.G in Community Medicine

MGMC&RI, Pondicherry

DefinitionCohort 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

• 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

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.

Design Of A Cohort Study

Population under study

Exposed

Develop outcome

Do not develop outcome

Unexposed

Develop outcome

Do not develop outcome

Framework of a cohort

CohortDisease 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

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.

Elements Of Cohort Study

Selection of study subjects

Obtaining data on exposure

Selection of comparison group

Follow up

Analysis

Types of Cohort Study

• Prospective cohort study

• Retrospective (historical) cohort study

• Combination of Retrospective and Prospective cohort

study

• Nested Cohort

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

Biases in cohort study

• Analytic bias

• Cross over bias

• Surveillance bias

• Information bias

• Loss to follow up bias

• Interviewer bias

• Non-response bias

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

Thank you