The outcome of interest was mortality or death.
And investigator may select a cohort specifically
to study certain uncommon or rare exposures.
In some cohort studies, population groups with known exposures to
a suspected hazardous substance or environment are first identified and
recruited for study.
And then another population or group without that exposure is identified.
And the risk or rate of that outcome over time is compared in the two groups.
For this reason, cohort studies can be
particularly useful for studying uncommon or rare exposures,
because usually it's possible to identify and assemble
groups of persons who have that uncommon exposure.
Other studies may create categories, such as amalgam
of risk factors for the disease or outcome under study.
The famous Framingham Cardiovascular Cohort Study provided much of the evidence
for what is known today regarding the risks of heart disease.
The study subjects were initially categorized according to
suspected risk factors, creating risk groups for comparison.
These risk groups were then followed for 20 to 30 years.
The development
of cardiovascular disease among the various risk
groups was then compared with statistical analysis.
A cohort study of US Air Force veterans from the Vietnam War was set up to
examine the effects of exposure to Agent Orange,
a defoliant dropped by planes during the campaign.
This group of veterans were compared to air force pilots active
at the time with no involvement in the Agent Orange campaign.
Attempting to conduct this study in the general population would have
not been possible as exposure to Agent Orange is too rare.
A common measure of health outcome occurrence in
a cohort study is a risk or a rate.
Since cohort studies are chosen to be free
of the outcome of interest at the outset, only
new health outcome events, such as diseases, behavior changes,
injuries, or even improvement in health status are considered.
Note that some cohort studies of diseased persons have been conducted.
For example, persons with arthritis.