What is a weakness of cohort studies?

• A. They may not be adequate for rare diseases/conditions
• B. They randomize participants
• C. They prove causation easily
• D. They are expensive

Answer: (A)

Cohort studies track people over time to see who develops an outcome based on their exposure, which helps establish that the exposure came before the outcome and lets you measure incidence. A key limitation is their efficiency when the outcome is rare: to observe enough events you’d need a very large group and long follow-up, making the study impractical and costly. That weakness is why cohort studies aren’t ideal for rare diseases or conditions.

Note: randomization isn’t part of cohort studies (that’s a feature of randomized trials), and while cohort studies can show associations, they don’t prove causation easily due to potential confounding. They can be expensive, but the most limiting issue for rare conditions is the need for enormous sample sizes to observe enough cases.