attrition bias

Attrition occurs when participants leave during a study. It almost always happens to some extent.

Different rates of loss to follow-up in the exposure groups, or losses of different types of participants, whether at similar or different frequencies, may change the characteristics of the groups, irrespective of the exposure or intervention. Losses may be influenced by such factors as unsatisfactory treatment efficacy or intolerable adverse events.

When participants leave, it may not be known whether they continue or discontinue an intervention; there may be no data on outcomes for these participants after that time.

Systematic differences between people who leave the study and those who continue can introduce bias into a study’s results – this is attrition bias. However, the results may not necessarily be biased, despite different drop-out rates in the groups. We discuss below how to assess the impact of different amounts of attrition.

In some cases, those who leave a study are likely to be different from those who continue. For instance, in an intervention study of diet in people with depression, those with more severe depression might find it harder to adhere to the diet regimen and therefore more likely to leave the study.