Gordis, L. (2008). Cohort Studies. In Epidemiology (4th ed., pp. 167-175). Philadephia, PA: Saunders Elsevier.

Cohort Studies Reference

A cohort study is designed to compare the incidence of a certain outcome between a group that has a certain characteristic and a group that lacks the characteristic. There are two basic ways of generating the study groups:
 - The groups are selected based on this characteristic from the start.
 - A defined population that was not selected based on this characteristic (i.e. a population that lives in a certain community) is separated into the study groups. This type of selecting a study population can allow investigators to observe multiple characteristics.


Designing a Cohort Study and its Study Population

A cohort study is one the best observational studies and is frequently used due to its ability to create a strong association between a phenomenon and an outcome.
 - A timeline can be established between the phenomenon and outcome, so it is also known that the phenomenon existed before the outcome
 - Identifies the incidence and risk of a new outcome
 - Able to assess several different outcomes

Strengths of Cohort Studies

One of the most well-known cohort studies, the Framingham Study looked at cardiovascular disease in the town of around 30,000 inhabitants in 1948. The study involved 5,127 men and women between the ages of 30 and 62 who did not have cardiovascular disease at the beginning of the study. Participants were followed up every 2 years while the hospital in Framingham kept note of daily hospitalizations, meaning it was a prospective cohort study.


Example of Cohort Study - Framingham Study

There are two types of cohort studies: prospective and retrospective. The only difference between the two types of cohort studies is the time frame as both still compare the group that has the factor and the group that does not.

Types of Cohort Studies

Depending on the study and hypothesis, a cohort study is not the best design that can be used. However, the biases included in this list can be avoided with good experimental design.

 - Expensive to conduct 
 - Prospective cohort study is often time-consuming
 - Inefficient for rare outcomes 
 - Selection bias 
 - Recall bias

Weaknesses of Cohort Studies

A cohort study is conducted to observe multiple groups of people over a period of time for comparison to see the potential effect of a phenomenon (possible influential factor) that the investigators are interested in.  The cohort studies are done either prospectively or retrospectively.  

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University of Michigan - Ann Arbor

An observational study, as supposed to an experimental study is a type of study that investigators observe the effect of an intervention without assigning it to the subjects at random.  That is, the subjects choose their own treatment/ intervention.  

Observational studies may prove to be less costly that RCT's, given that the investigator need not actively change the behavior of the subjects


Observational Studies

Pearl, J., & Mackenzie, D. (2018). $\mathit{The \ Book \ of \ Why: \ The \ New \ Science \ of \ Cause \ and \ Effect.}$ Basic Books.

The Book of Why

Cohort Studies

If substance A leads to biological reaction B, then in general (but not 100%), a larger dose of A leads to the stronger reaction B. 


Dose-response effect

Case-control studies are observational studies where subjects' previous experiences are used as evidence to assess how those who had experience with the specified condition varied from those who did not and why.

Case-control studies

-Observational studies can be conducted in the natural habitat of the target population as opposed to an artificial, controlled setting in laboratory
-Observational studies maintain "purity" in that there are few issues of ethics or feasibility due to no need for intervention being applied. For example, if investigating smokers and health, we cannot make non-smokers start smoking for the experiment, but we can find people who are already smokers. 

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