Which bias refers to systematic misclassification of exposure or outcome?

• A. Loss to follow-up
• B. Classification bias
• C. Confounding bias
• D. Selection bias

Answer: (B)

Misclassification bias occurs when exposure or outcome is measured or classified inaccurately in a systematic way. When this kind of error consistently mislabels who is exposed versus unexposed or who has the outcome, it can distort the observed association between exposure and outcome. This specific type of bias is called classification bias, since it stems from how the key variables are classified rather than from the study design itself.

Other biases come from different issues: loss to follow-up (attrition bias) happens when participants drop out in a way that depends on exposure or outcome; selection bias arises from non-random selection of participants; confounding bias occurs when a separate factor is related to both the exposure and the outcome and is not adequately controlled.