[Critical assessment of articles on controlled clinical trials. I. Planning]

This is the first of two articles concerning the techniques of critical appraisal of controlled clinical trials. It presents an overview of the delicate points of the following issues: operational questions; definition of primary and secondary variables, classification of the trial as pragmatic or explanatory, selection of the population sample, study design and its appropriateness to the goals, sample size and overall statistical analysis. Particular attention is given to the value of randomisation and blinding for the prevention of bias.