Evidence-appraisal glossary
Ascertainment bias
Ascertainment bias is systematic error arising from the way cases, outcomes, or data are identified, so that some are more likely to be captured than others. The result is a distorted picture that does not reflect the true underlying pattern.
Also called: sampling bias.
Ascertainment bias appears when detection depends on factors linked to exposure or group, for example a monitored group having more tests and therefore more diagnoses than an unmonitored one. The term overlaps heavily with detection bias and surveillance bias, and usage varies between fields, so it is worth checking how a given paper defines it. The core issue is unequal opportunity to be counted, not a true difference in how often events occur.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.