Stopping guidelines vs stopping rules: a practitioner's point of view

Monitoring interim accumulating data in a clinical trial for evidence of therapeutic benefit or toxicity is a frequent policy, usually carried out by an independent scientific committee. While statistical methodology has been developed to assess the significance of these interim analyses, such methods should not be viewed as absolute rules but only serve as useful guides. The decision process to terminate a trial early is very complex and many factors must be taken into account. The complexity of this decision process is illustrated by reviewing the experience of several recent clinical trials.