Studies of drugs given before anaesthesia. XV. Evaluation of the method of study after 10,000 observations.

SUMMARY The method used in a long-continuing study of drugs given as premedication to healthy patients undergoing minor gynaecological operations has been reviewed in order to evaluate the importance of various potential sources of bias. Factors which were found not to have influenced the results, in the context of the standard method used throughout the study, included the subject's body weight and age, and the use of different observers. Those factors which did affect the outcome included the frequency of pre-operative visits of the subject by the observer, the precise nature of the operation performed, and the duration of anaesthesia. These considerations are discussed in the light of the design of similar clinical trials.