Retention

Most trials involve more than one contact with a participant after they have been recruited, allowing for data collection over a pre-defined period of time. This contact comes in many forms; some examples include face-to-face meetings, through letters, the internet, telephone calls, text messaging, email or the return of medical test kits. Missing data can result from any of these data collection methods, sometimes people are too busy to reply, they have changed their mind and no longer want to participate in the study, or they may have experienced unwanted or unexpected side effects as a result of trial participation.

The attrition of trial participants can impact negatively on the trial’s results. Bias can be introduced if participants drop-out as a result of experiencing severe side effects, and the study can be underpowered if attrition rates are substantial.