American Board of Surgery Qualifying Exam (ABS QE) Practice Test

What type of bias occurs due to differences in assessing outcomes based on knowledge of group assignments?

1. Ascertainment bias

2. Confounding bias

3. Lead time bias

4. Follow-up bias

The correct answer is: Ascertainment bias

Ascertainment bias occurs when the outcomes of a study are assessed differently based on the knowledge of which participants are in which treatment group. This can happen, for example, if researchers or evaluators have preconceived notions about the effectiveness of a treatment and allow that knowledge to influence how they interpret the outcomes. The result could lead to skewed data, as the information may be interpreted in a way that favors one treatment over another, compromising the objectivity of the assessment. Confounding bias refers to errors in estimating the effect of an exposure or treatment due to the presence of a third variable that is associated with both the exposure and the outcome, not necessarily tied to knowledge of group assignments. Lead time bias is related to the time gap introduced by earlier detection of a disease due to screening, while follow-up bias occurs when there are systematic differences in follow-up among study participants based on their treatment or outcomes. Each of these biases addresses different aspects of study design and outcome assessment, highlighting how ascertainment bias is specifically linked to knowledge of group assignments.