Background
RECRUITMENT
Completeness of follow-up data is vital to trial validity. Despite this, most trials have non-trivial levels of missing data (called ‘loss to follow-up’ or ‘attrition’) [There are many different methods that have been suggested or are used to improve retention rates in clinical trials, especially where the follow-up is through participant-completed questionnaires, but reports of the effectiveness of many of these are anecdotal [We therefore set out to examine the effect of adding in a monetary incentive of £20 as either conditional (given only once their questionnaire is returned) or non-conditional (given to all participants at the point of recruitment) as a sub-study nested within a large international randomised controlled trial, the Antivirals for influenza-Like Illness? An rCt of Clinical and Cost effectiveness in primary CarE (ALIC