Which is a common bias in cohort studies?

• A. Selection bias only
• B. Information bias only
• C. Loss to follow-up
• D. Confirmation bias only

Answer: (C)

Loss to follow-up is a key concern in cohort studies because these designs rely on tracking the same group of people over time to observe who develops the outcome. When participants drop out, especially if dropout is related to exposure or disease risk, the remaining group becomes unrepresentative of the original cohort. If those who leave are more likely to experience the outcome (or have a different exposure pattern), the estimated association between exposure and outcome can be biased. This attrition bias can distort measures like incidence and risk ratios and also reduce study power due to smaller sample sizes. While other biases like selection bias, information bias, or cognitive biases such as confirmation bias can occur in research, loss to follow-up is the most characteristic and impactful bias in the context of following people over time in a cohort study.