Time consumption is an issue in prospective cohort studies as the investigator must reach out to the participants over a time frame whereas in retrospective studies, the information is already recorded. If the follow-up period is extremely long for a study, this could result in a series of issues. 
 - The study participants may outlive the investigator.
 - Non-response bias - Participants may begin to leave the study or may not be contactable; Participants who left the study may have different outcomes than those who stayed, resulting in a loss of data

Weakness of Time Frame in Prospective Cohort Studies

Rare outcomes would require an incredibly large study population. They can also have a long latency period, which could result in having a  follow-up period, which introduces its own problems. 

Weakness of Cohort Studies with Rare Outcomes

Recall bias is a error that occurs when participants do not remember previous events or experiences accurately or omit details: the accuracy and volume of memories may be influenced by subsequent events and experiences.

Recall Bias

Sampling bias is a systematic error that occurs when the method used to select participants results in a sample that fails to accurately represent the broader target population. Because the selected individuals differ in important ways from the overall group, any statistical findings derived from this unrepresentative sample will likely be inaccurate and cannot be safely generalized.

Sampling Bias

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

University of Pittsburgh

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.  

Cohort Studies

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.

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