Control of error in randomized clinical trials

The randomized clinical trial is the method of choice for comparing the eff ects of alternative care options, both in its own right and as the cornerst one of systematic reviews of the subject. Errors in such trials, therefore, have major consequences for health care. This paper provides a brief introduction to the major sources of such error s, whether they be systematic or chance errors. It addresses selection bias , due to either biased entry in or biased exclusion from the trial, bias in assessing outcomes, and biases due to contamination or co-intervention. Ra ndom errors, including type I and type II errors, are discussed along with ways in which they can be minimized. Small clinical trials, in particular, provide a major problem not only by themselves, but also if they become inc orporated in systematic reviews without appropriate consideration of the ph enomenon of publication bias. (C) 2000 Published by Elsevier Science Irelan d Ltd.