Attrition bias

A bias associated with the dropping out or the differential exclusion of subjects from a study.

Note: If subjects drop out of the study for some reason, their exclusion from the analysis could lead to an overestimate of the effectiveness of the intervention. The differential dropping out of subjects may be due to a selection bias in a comparative experimental study or cohort observation study.

Reference

http://htaglossary.net/attrition-bias