Evidently Cochrane's new blog about participant retention gives us some interesting food for thought - sample below, and link to full text:
What do we know about the best ways to keep people involved in clinical trials? Frankly, not a lot.
Clinical trials can provide evidence about which treatments work. Clinical trials are research studies that involve people and compare different groups of people receiving different treatments and look at which treatments improve outcomes (like pain) the most. During the design of a trial, calculations are made about how many people need to join the trial and complete all of the data collection requirements (which might be, for example, a questionnaire or a clinic appointment). These calculations allow the researchers to be confident in the results at the end of the trial.