In this presentation i tried to explain in detail about cohort studies, their types, how to conduct them, their outcomes, and how to calculate sample size of these studies.

1. 6.5 out of 10 people
remember what they see…
A Presentation on Cohort studies
By: Dr. Ankit Gaur
M.Sc, Pharm.D, RPh

2. DIFFERENT STUDY TYPES
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3. Cohort Study
Cohort study
It is an type of observational study:
• It begins by identifying subjects (the cohort) with a common trait such as a disease or risk factor
•observes a cohort over time
• can be conducted retrospectively or prospectively

4. A retrospective study uses existing data to identify a population and exposure status. Since we are looking back
in time both the exposure and outcome have already occurred before the start of the investigation. It may be
difficult to go back in time and find the required data on exposure, as any data collected was not designed to be
used as part of a study. However, in cases where reliable records are on-hand, retrospective cohort studies can
be useful.

5. In a prospective cohort study, the researcher identifies subjects comprising a cohort and their exposure status at the
beginning of the study. They are followed over time to see whether the outcome (disease) develops or not. This
usually allows for better data collection, as the actual data collection tools are in place, with required data clearly
defined.

6. In cohort studies healthy subjects with or without exposure to some risk factor are observed over a time
period to see the event rate in both groups.
If a researcher wants to see the impact of weight training exercise on cardiovascular mortality then he will
select two groups, one consisting of subjects who do exercise and another consisting of those who don’t do.
 These groups will be followed up for a specific time period to see cardiovascular mortality in both groups.
At the end of the study period both groups will be compared for cardiovascular mortality.
The formula for sample size is

7. So suppose the researcher wants to see the impact of weight training exercise on cardiovascular mortality and according
to previous studies proportion of cardiovascular death in case may be around 20% and in control it can be around 40%
hence sample size calculation for 5% of significant level and 80% power with equal number of case and control will be
So, the researcher has to take 59 samples in each group.
It is worthy of mention here that these formulas for case control and cohort study are for independent
design studies. They are not for matched case control and cohort studies.
 These formulae can be modified or corrected depending on population size or ratio between sample size
and population size .
Use various freely available epidemiological calculators like openEpi to calculate sample size formula.

8. Absolute risk (incidence)
Relative risk (risk ratio or rate ratio)
Risk difference
Attributable proportion (attributable risk %)

9. ADVANTAGES:
 Gather data regarding sequence of events; can assess causality.
Examine multiple outcomes for a given exposure.
Good for investigating rare exposures.
Can calculate rates of disease in exposed and unexposed individuals over time (e.g.
incidence, relative risk).
DISADVANGATES:
At the starting of study
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Loss in middle of
study…