Evidence-appraisal glossary
Detection bias
Detection bias is systematic error from measuring, looking for, or confirming an outcome more thoroughly in one comparison group than another. When the exposed or treated group is watched, tested, or scored differently, the recorded difference in outcomes partly reflects unequal detection rather than a real effect.
Also called: ascertainment bias, assessment bias, surveillance bias.
What it is
Detection bias (also called ascertainment or assessment bias) arises when the way an outcome is sought, measured, or verified differs systematically between the groups being compared. If clinicians order more tests for treated patients, or an unblinded assessor scores a subjective endpoint more favorably in one arm, some of the observed difference comes from unequal detection, not a true effect.
How to use it when reading a study
- Check who assessed outcomes and whether they were blinded. Blinded outcome assessment is the main defense, especially for subjective endpoints (pain, disability, cause of death).
- Look for equal surveillance. Were both groups tested, imaged, and followed up on the same schedule? Extra monitoring of one group inflates its apparent event rate.
- Judge outcome type. Hard, objective endpoints (all-cause mortality) resist detection bias; investigator-judged or diagnosed outcomes are vulnerable.
- Watch observational studies. Sicker or treated patients often get more scrutiny, manufacturing spurious associations.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.