Bias in selecting and retaining participants in a study is called:

Correct answer: B. Selection bias

• A. Incorrect. Observer bias (ascertainment bias) occurs when the researcher's knowledge of group assignment affects outcome measurement.
• B. Correct. SELECTION BIAS is the systematic error that occurs when participants are chosen or retained in a way that makes the study population unrepresentative of the target population. It affects both who is initially enrolled AND who continues to participate (attrition/loss to follow-up).
• C. Incorrect. Sampling bias specifically refers to non-random selection of the sample from the population — a type of selection bias but more specific.
• D. Incorrect. Confounding bias occurs when a third variable influences both the exposure and outcome, distorting the apparent association.