Evidence-appraisal glossary
Attrition bias
Attrition bias is systematic error introduced when participants who drop out or are lost to follow-up differ from those who remain, and the loss is unequal or related to outcome. Intention-to-treat analysis limits it but does not fully cure it.
Also called: dropout bias.
Attrition bias matters when the reasons for leaving a study are tied to the treatment or the outcome, such as side effects driving sicker patients out of one arm. Analyzing only completers can then paint a falsely favorable picture. High dropout does not guarantee bias, and low dropout does not guarantee its absence; what matters is whether the missing data differ systematically between groups.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.