Loss to follow-up in a cohort study can lead to which issue?

• A. Confirmation bias
• B. Mutation bias
• C. Attrition bias
• D. Selection bias

Answer: (C)

Attrition bias is the key idea here. When some participants drop out of a cohort study and the reasons they drop out are related to both their exposure and their risk of the outcome, the remaining group isn’t representative. This can systematically distort the estimated association between exposure and disease. For example, if heavier smokers are more likely to leave the study, the observed link between smoking and lung cancer among those who stay may be weaker than the true link.

This isn’t about investigators’ thinking (that would be confirmation bias), nor is it about random genetic change (mutation bias). It’s specifically about loss to follow-up changing the composition of the study population over time, which is why it’s termed attrition bias. Selection bias is a broader idea about how participants are chosen or retained, but the precise, standard label for bias caused by differential loss to follow-up is attrition bias.