“If it were not for the great variability among individuals, Medicine might be a Science, not an Art”—Sir William Osler, 1882, The Principles and Practice of Medicine

It is important to apply current best evidence in making decisions about management of individual patients. While the evidence may be derived from basic and applied research, the findings from large scale clinical trials of interventions are the most relevant. However, in many cases there are uncertainties around the effects of treatments and indeed guidelines can “legitimise” these uncertainties by defining boundaries within which decisions are reasonable. Therefore, the appropriate interpretation of clinical trial results is just as important for those who are charged with the development and implementation of guidelines as they are for the clinician in discussing options with individual patients.

Important aspects relating to trial design and interpretation are discussed, using illustrative examples drawn from various fields of cardiovascular medicine.

The science of clinical trial methodology has been discussed in detail elsewhere,1 and the application of trial results to individual patients considered by other authors,2including overviews of trials of many interventions. However, to date, relatively few relating to cardiovascular medicine have been produced through the Cochrane Collaboration (http://www.epi.bris. ac.uk/cochrane.heart.htm).3