Abstract
Studies that investigate the usefulness of various therapies fall along a continuum that ranges from those looking at whether an intervention can work under ideal circumstances (efficacy trials) to those that focus on whether a treatment works when applied in the real world (effectiveness trials). Whether a study is closer to one end of the spectrum or the other affects almost every aspect of the trial. These aspects include which patients are eligible for enrolment, the degree of control over the way the intervention is delivered, which patients are or are not included in the analyses, how missing data are handled, and even which statistical tests may be used. The 2 types of trials may yield different results, but both provide useful information. This paper explores these issues, shows the decisions researchers must take at each phase of a trial, and discusses how clinicians should interpret the results.