Attrition is the loss of participants during a clinical trial it is also known as the 'drop-out rate'. The opposite of attrition is 'retention'.
Attrition can cause bias in study results if more participants drop out of one study arm than another, or if there is a difference between the participants who drop out and those who continue. Attrition can affect how applicable the results of a study are (external validity), or the statistical power of a study.
Research suggests that using a combination of strategies can improve the retention of participants in trials. Many different methods may be appropriate, for example providing incentives for taking part, giving personal reminders of appointments, and involving patients and/or their carers throughout the trial-design process.