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. 

University of Pittsburgh

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

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

Cohort Studies Reference

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

Weakness of Cohort Studies with Rare Outcomes

Selection bias is the bias introduced by the selection of individuals, groups or data for analysis in such a way that proper randomization is not achieved, thereby ensuring that the sample obtained is not representative of the population intended to be analyzed.

Selection bias

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.

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