This document outlines the design and conduct of cohort studies. It defines a cohort study as selecting exposed and unexposed groups and following them over time to compare disease incidence. Key points include: cohort studies can establish temporal relationships between exposures and outcomes; they calculate relative risk to measure effects; and they can be prospective, retrospective, or combine features of both. Biases, reporting, and critical appraisal of cohort studies are also discussed.

2. Learning Objectives
• To describe the designs of cohort studies .
• Options for the conduct of cohort studies.
• To discuss some potential biases in cohort studies.
• To calculate and interpret relative risk and other measures of effects
in a cohort study.
• How to do reporting of a cohort study?
• How to critically appraise a cohort study?

3. DESIGN OF COHORT STUDY
oThe investigator selects a group of exposed individuals and a group of
unexposed individuals and follows both groups over time to compare
the incidence of disease in the two groups.
oCohorts may be identified retrospectively or prospectively, but in
either case the outcome status needs to be established at least twice.
oIt must be established that a cohort did not have the outcome of
interest at the beginning of the observation period, and the cohort
needs to be examined again to determine whether or not the
outcome subsequently developed.

4. Design of a cohort study
Disease
develops
Disease
does not
develop
EXPOSED
NOT
EXPOSED
Disease
develops
Disease
does not
develop
&

5. • Because we are identifying new (incident) cases of disease as they
occur, we can determine whether a temporal relationship exists
between the exposure and the disease (i.e., whether the exposure
preceded the onset of the disease).
• a temporal relationship must be established if we are to consider the
exposure a possible cause of the disease in question.

6. Disease No Disease Total Incidence
EXPOSED a b a+b I(E) = a/a+b
NOT EXPOSED c d c+d I(NE)= c/c+d
If the exposure is associated with disease, the incidence rate is greater
in the exposed group than in the unexposed group.

7. • A closed cohort is one with fixed membership.
• Once the cohort is defined by enrolling subjects and follow up begins,
no one can be added.
• The number of subjects may decline because of death or loss to follow
up, but no additional subjects are added.
• As a result, closed cohorts gets smaller over time.
• E.g. Citizens of Japan who were exposed to radiation when atomic
bombs were dropped on Hiroshima and Nagasaki during the second
World War, would be considered members of a fixed or closed cohort
that was defined by an event.
Types of Cohorts

8. • An open cohort is dynamic, meaning that members can leave or be
added over time.
• E.g. a state cancer registry. Subjects are continually added when they
are diagnosed with cancer, so new subjects are continually added.
Subjects can also leave the cohort by moving to a new state or dying.

9. When to select a Cohort design?
• When the aim of the study is to investigate risk factors or prognostic
factors.
• The disease is reasonably common one.
• The follow-up period required is reasonably short so that the study can
be conveniently completed within the time frame.
• Some amount of evidence regarding the association between exposure
and outcome has already been provided by case control or cross
sectional studies.
• There is reasonable likelihood of the subjects in the two groups
continuing to participate in the study.

10. Retrospective Cohort Study
• The study design does not differ from that of the prospective
cohort design—we are comparing exposed and unexposed
groups.
• In a retrospective cohort design, exposure is ascertained from
past records and the outcome is determined when the study is
begun.
• Use historical data so that we can telescope (reduce) the frame of
calendar time for the study and obtain our results sooner.

12. Ambidirectional Cohort Study
• It is also possible to conduct a study that is a combination of
prospective cohort and retrospective cohort designs.
• With this approach, exposure is ascertained from objective records in
the past (as in a historical cohort study) and follow-up and
measurement of outcome continue into the future.

13. Case-Control Studies Based Within a Defined Cohort
• Taking advantage of the benefits
of both case-control and cohort
study designs by combining
some elements of both into a
single study.
• The resulting combined study is
in effect a hybrid design in
which a case-control study is
initiated within a cohort study.

14. NESTED CASE CONTROL STUDIES
• At the time each case or cases
develop, the same number of
controls is selected.
• In this way, the cases and controls
are, in effect, matched on calendar
time and length of follow-up.

15. CASE-COHORT STUDIES
• Here the controls are randomly
chosen from the defined cohort
with which the study began.
• Advantage : it is possible to study
different diseases (different sets of
cases) in the same case-cohort
study using the same cohort for
controls.
• Disadvantage: A case may also be
selected as control

16. Advantages:
Laboratory tests would not need to be performed on all people in the
original cohort. Thus the laboratory burden and costs are dramatically
reduced.
the problem of possible recall bias is eliminated.
In both nested case-control and case-cohort designs, cases and
controls are derived from the same original cohort, so there is likely
to be greater comparability between the cases and the controls than
one might ordinarily find in a traditional case-control study.

17. References
• Bhalwar R.Textbook of Community medicine,3e. Wolters Kluwer India
Pvt.Ltd.2019
• Gordis Epidemology.David D Celentano,Moyes Szklo;6th edition