Stopping clinical trials by design.

Before any clinical trial begins, a detailed trial protocol must be prepared. The authority of the trial results will depend on the quality of this document. In many protocols, a key component is a plan for a series of interim analyses of the accumulating trial data, and a 'stopping rule' based on them. Such a rule might be intended to prevent participants from continuing to receive a drug that already seems to be unsafe, or to allow a successful drug to become generally available as soon as sufficient evidence of its advantages has been collected. There has been considerable misunderstanding of these rules, and controversies associated with them. Here, I discuss why this might be, and what can be done to promote their successful and beneficial use in the future.