This is the fourth of an occasional series on the methods of randomised controlled trials

When patients do not receive their preferred treatment in randomised trials there may be difficulties with patient recruitment and scientific problems with bias.1 For example, bias may occur when patients are aware of a new treatment not available to them and comply poorly with the standard treatment.

Zelen's design can address these difficulties 2 3 by randomising patients before consent to participate has been sought. Two types of the design exist: double and single consent. In the double consent version patients are initially offered the treatment to which they were randomised; however, if they decline the randomised treatment, they can then be offered alternative therapies—including the experimental treatment. In …