A cohort study follows groups of individuals over time who differ by whether they have been exposed or not exposed to a particular risk factor or intervention. Researchers then compare how often the outcome of interest occurs in each group. Cohort studies begin by identifying groups of individuals and following them prospectively to determine what exposures may be associated with outcomes. They allow for standardization of criteria and outcomes and can be easier than randomized controlled trials. However, confounding variables can be difficult to control for without randomization.

1. PRATIGYA DEUJA

2.  Cohort studies, also called as follow-up or incidence studies,
begin with a group of people who are free of disease and who
are classified into subgroups according to exposure to a
potential cause of disease or outcome.
 Definition: A study design where one or more samples (called
cohorts) are followed prospectively and subsequent status
evaluations with respect to a disease or outcome are
conducted to determine which initial participants exposure
characteristics (risk factors) are associated with it. As the
study is conducted, outcome from participants in each cohort
is measured and relationships with specific characteristics
determined.

3.  Subjects in cohorts can be matched, which limits the
influence of confounding variables.
 Standardization of criteria/outcome is possible.
 Easier and cheaper then a randomized control trial (RCT).

4.  Cohorts can be difficult to identify due to confounding
variables.
 No randomization, which means that imbalances in patient
characteristics could exist.
 Blinding/masking is difficult.
 Outcome of interest could take time to occur.

5.  Cohort studies is to work from “cause to effect”.
 In a cohort study, the exposure has occurred, but the disease
has not.
 For an instance, group or cohort (a+b) exposed to a particular
factor thought to be related to disease occurrence, and a
group (c+d) not exposed to that particular factor. The former
is known as “study cohort” and the latter “control cohort”.

6.  The groups are then followed, under the same identical
conditions, over a period of time to determine the outcome
of exposure
(e.g, onset of disease , disability or death) in both the groups.
In chronic diseases such as cancer the time required for the
follow up may be very long.

7.  (a+b) persons were exposed to the factor under study, ‘a’ of
which developed the disease during the follow-up period.
 (c+d) persons were not exposed, ‘c’ of which became cases.
 After the end of the follow-up, the incidence rate of the
disease in both the groups is determined.
 If it is found that the incidence of the disease in the exposed
group, a/(a+b) is significantly higher than in the non-exposed
group, c/(c+d), it would suggest that the disease and
suspected cause are associated. This prospective approach is
known as cohort studies.

8.  For example; The long term effects of exposure to
uranium was evaluated by identifying a group of uranium
miners and a comparison group of individuals not
exposed to uranium mining and by assessing subsequent
development of lung cancer in both groups.
 The principal finding was that the uranium miners had an
excess frequency of lung cancer exposed to non-miners.
Since the disease had not yet occurred when the study
was undertaken, this was a prospective cohort design.

9.  A well designed cohort study is considered the most reliable
means of showing an association between a suspected
disease because it eliminates many of the problems of the
case control study and approximates the experimental model
of the physical sciences.