When original participants in a longitudinal study drop out midway, it is called

When original participants drop out midway through a longitudinal study, the loss is termed mortality or attrition.

Experimental distancing is not a recognised methodological term for participant loss.

Respondent deviation does not name the dropping-out of panel members over time.

Sample redundancy refers to surplus cases, not to those who leave the study.

Heavy mortality threatens validity because remaining participants may differ systematically from those who left.

Panel or cohort studies following the same people are especially vulnerable to this attrition.