Which statement is true about bias in cohort studies compared to case-control studies?

• A. Case-control studies are never subject to selection bias.
• B. Cohort studies measure exposure before outcome, reducing recall bias.
• C. Case-control studies are more efficient for rare outcomes.
• D. Cohort studies are more prone to recall bias than case-control.

Answer: (B)

In cohort studies, exposure information is collected before the outcome occurs, often in real time or from records. This timing means people aren’t influenced by knowing whether they developed the disease, so their memory of past exposures isn’t biased by their disease status. That minimizes recall bias, a common problem when people with a disease remember past exposures differently from those without the disease.

In contrast, case-control studies gather exposure data after disease has occurred, frequently relying on participants’ recall. Cases may think back differently about what they were exposed to, or recall exposure inaccurately because of their current health status, leading to recall bias and distorted associations.

So the statement that exposure is measured before the outcome in cohort studies, thereby reducing recall bias, best captures how bias differs between these designs. Other statements either focus on efficiency for rare outcomes (which relates to study design practicality rather than bias) or misstate the bias risk, as selection bias can occur in both designs and recall bias is not inherently greater in cohort studies.