Every clinical trial is an experiment which aims to improve knowledge about a new treatment for patients. The question to be answered may be quite simply ‘does it work?’ or ‘is it safe?’. Usually doctors want to be more precise than this, so it is common to ask ‘is this new drug better than the old one, either in terms of treating the disease better or quicker, or in terms of the same benefits but with fewer side-effects?’. There are many different ways to test the risks and benefits of new treatments; the trick is to choose the design which gives you an answer to the question as quickly as possible and involves the minimum number of patients in the experiment. The design of the trial is a description of the way patients will be studied, in terms of selection, treatment and assessment.