ABSTRACT
A randomized clinical trial is a prospective experiment to compare one or more interventions against a control group, in order to determine the effectiveness of the interventions. A clinical trial may compare the value of a drug vs. placebo. It may compare surgical with medical interventions. The principles apply to any situation in which the issue of who is exposed to which condition is under the control of the experimenter, and that the method of assignment is through randomization. A negative clinical trial is that in which no significant difference is found between the comparison groups. Results without statistical difference may be useful either to discard useless treatments or to demonstrate that one intervention is as effective as the one it was compared with. Eliminating useless treatments may be adequate. However, if this is the result of studies with methodological errors, new interventions that are actually useful may not be available for patients. In this review we present some of the possible methodological errors that may lead to false negative results in clinical trials.