Evidence-appraisal glossary
Loss to follow-up
Loss to follow-up is when participants enrolled in a study drop out or cannot be reached before their final outcome is measured, so their results are missing. If those who leave differ systematically from those who stay, the missing data can distort the study's conclusions.
Also called: Attrition, Dropout, Study withdrawal.
What it is
Every trial or cohort study loses some participants along the way: people move, withdraw consent, feel better or worse, or simply stop showing up. When their outcomes are never recorded, those data are missing. The concern is not the missingness itself but whether the people who left differ from those who stayed in ways tied to the outcome, which produces attrition bias.
How to use it when reading a study
- Check the flow diagram: how many were randomized versus analyzed, and why participants dropped out.
- Compare arms. Bias is more likely when the amount of loss, or the reasons for it, differ between the treatment and control groups (differential attrition).
- Use rough anchors cautiously: under 5 percent loss is usually reassuring, over 20 percent should raise real concern, but the reasons matter more than the percentage.
- See how authors handled missing data (for example intention-to-treat, imputation, sensitivity analyses). Complete-case analysis alone can flatter a treatment.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.