A cohort study follows groups of individuals (cohorts) over time to examine the effects of an exposure. Cohorts are defined based on their exposure status. The exposed cohort has the characteristic/exposure being studied, while the unexposed cohort does not. Researchers observe the cohorts for the occurrence of outcomes and calculate measures like relative risk to quantify associations between exposure and outcome. Prospective cohort studies follow cohorts forward in time, while retrospective studies use existing data to look back. Cohort studies are useful for examining multiple outcomes simultaneously and estimating risks but require large samples and long follow-up periods.

1. COHORT STUDY

2. OVERVIEW

3. INTRODUCTION
• COHORT – Cohort is a group of persons
possessing common characteristics.
• Example – Those born during a particular
time, constitute a birth cohort, those exposed
to a common vaccine or drug are called
exposure cohort.

4. KEY TERMINOLOGIES
Exposure
• A supposed cause of disease, health state of
interest, or characteristic that is a determinant of
the health outcome of interest.
Classifications:
• Exposed group: Has the characteristic of interest
• Unexposed group: Does not have the
characteristic and thus serves as the control
group

5. KEY TERMINOLOGIES
Outcome
An event of interest that may be influenced by
the exposure. Those with the outcome are
compared to those without the outcome.
Classification:
Has met the well defined criteria for the
incidence of the disease or health outcome of
interest.

6. STUDY DESIGN
Cohort Studies are observational, analytical
studies.
• Observational because the investigator follows
the exposure of interest but does not provide
an intervention.
• Analytical because the intent is to identify and
quantify associations, test hypotheses, and
identify causes.

7. WHEN TO USE A COHORT STUDY
DESIGN
• Some evidence between exposure and
disease.
• Exposure rare but incidence among cohorts is
high
• Time between exposure and disease is short
Resources available

8. TYPES OF COHORT STUDY
There are three types of cohort studies namely,
• Prospective cohort study
• Retrospective cohort study
• Combination of Prospective and Retrospective
cohort study

9. PROSPECTIVE COHORT STUDY
Prospective cohort study:
• Investigator starts the study (from the
beginning) with the identification of the
population and the exposure status
(exposed/not exposed groups)
• Follows them (over time) for the development
of disease
• Takes a relatively long time to complete the
study (as long as the length of the study)

10. RESTROSPECTIVE COHORT STUDY
Retrospective cohort study:
• Investigator uses existing data collected in the
past to identify the population and the
exposure status (exposed/not exposed
groups)
• Determines at present the (development)
status of disease

11. RESTROSPECTIVE COHORT STUDY
Investigator spends a relatively short time to:
• Assemble study population (and the
exposed/not exposed groups) from past data
• Determine disease status at the present time
(no future follow-up)

12. COMBINED PROSPECTIVE AND
RETROSPECTIVE STUDY
Investigator uses existing data collected in the
past to,
• Identify the population and the exposure
status (exposed/not exposed groups)
• Follow them into the future for the
development of the disease

13. COMBINED PROSPECTIVE AND
RETROSPECTIVE STUDY
Investigator,
• Spends a relatively short time to assemble
study population (and the exposed/not
exposed groups) from past data
• Will spend additional time following them into
the future for the development of disease

14. Type of study The Past The Present The Future
Prospective
(longitudinal)
Cohort study
Retrospective
Study
Case control
Study
Cross-sectional
study
Assemble cohorts in the present
and collect data on present risk
factors
Assemble cohorts in the present
based on past risk factors and
collect data on present
outcomes
Assemble cases and controls in
the present based on present
outcomes, and collect data on
past risk factors.
Associations between present
exposures and present
outcomes
Collect data at a time
in the future on
outcomes that arise
When exposure
occur defines
cohorts
When exposure
occu that may be
associated with
outcomes

15. STUDY DESIGN – COHORT STUDY
Define cohort, create eligibility criteria,
recruit subjects free of the outcome
Determine exposure status (administer
survey, obtain samples, measurements,
etc.)
Follow subjects forward in time to
determine incidence of the outcome (may
be followed in historical documents or in
real time)

16. STEPS IN A COHORT STUDY
• SELECTION OF STUDY COHORTS
• SELECTION OF CONTROL COHORTS
• FOLLOW-UP
• ANALYSIS

17. FRAMEWORK OF COHORT STUDY
Cohort Group Development of
disease
Total
Yes No
Study Cohort
(Exposed to
suspected factor)
a b a+b
Control cohort
(Not exposed to
suspected factor)
c d c+d

18. INDICATORS IN COHORT STUDY
• Relative risk
• Risk difference
• Attributable risk
• Absolute risk
• Population attributable risk
• Population attributable risk population

19. COHORT STUDY ANALYSIS
RR is measure of association
between incidence of
disease and certain exposure
Relative risk (RR) answers
the question
How many times a person
who is exposed is at risk of
developing disease
compared to non-exposed?‖
Quantify risk

20. CALCULATING RELATIVE RISK
The basic steps in the calculation of a relative
risk
1. Incidence rates among the exposed = (a/a+b)
2. Incidence rates among the non-exposed =
(c/c+d )
RELATIVE RISK = Incidence among exposed
Incidence among non - exposed

21. INTERPRETATION OF RELATIVE RISK
• RR=1.0 – No association between exposure
and disease incidence rates. Rates are
identical between groups.
• RR> 1.0 – Positive association (increased risk)
exposed group has higher incidence than non-
exposed group
• RR < 1.0 – Negative association (protective
effect) non-exposed group has higher
incidence than exposed group

22. RISK DIFFERENCE
• Risk difference is the mathematical difference
between the incidence of the disease among
exposed and unexposed.
RD = IE - IO

23. ATTRIBUTABLE RISK
• Attributable risk is the ratio of risk difference
and the incidence of disease among exposed,
often expressed in percentage.
AR = RD
IE

24. ABSOLUTE RISK
• It is the risk of developing the disease,
irrespective of exposure to the risk factor and
is expressed as percentage.
ABSOLUTE RISK = a+c
x 100
a+b+c+d

25. POPULATION ATTRIBUTABLE RISK
• Population attributable risk is the difference
between the incidence of the disease occuring
in the total population of both the groups
irrespective of risk factor and incidence of
disease among the control group.
PAR = IP - IO

26. BIAS IN COHORT STUDY
Selection bias
• Select participants into exposed and not exposed
groups based on some characteristics that may affect
the outcome „
Information bias
• Collect different quality and extent of information from
exposed and not exposed groups
• Loss to follow-up differs between exposed and not
exposed (or between disease and no disease) „
Misclassification bias
• Misclassify exposure status or disease status

27. ADVANTAGES OF COHORT STUDY
• Provides information about true incidence rates
• Several other outcomes can be studied
simultaneously
• Helps to estimate relative risk and attributable
risk
• Allows the assessment of dose-response
relationship
• Helps to accept or refute the hypothesis with a
high degree of validity

28. DISADVANTAGES OF COHORT STUDY
• Time consuming
• May require very large samples
• Expensive
• Not suitable for rare diseases
• Not suitable for diseases with long-latency
• Unexpected environmental changes may
influence the association
• Non-responsive migration and loss-to-follow-up
biases