[Analysis of the results of the trial where groups have been randomized. The Paris cardiovascular prevention trial (author's transl)].

In a prevention trial, when the randomised units are groups of subjects, the first step in the analysis of the results is to check whether the groups differ, for the variables under study, ie, to test a possible group effect. If this effect is not significant, the results are analysed as if the subjects had been randomised. On the other hand, it this effect is significant, the comparison must be carried out between the groups and no more between the subjects. In the latter case, the loss of efficiency of the randomization of groups instead of the randomization of subjects can be computed. When a group effect is present and the number of subjects in each group differs, the analysis is considerably more complex. In the Paris cardiovascular prevention trial, 160 groups of young men, with variable numbers of subjects in each group have been randomised. The change in weight, blood cholesterol and cigarette consumption after two years of intervention are analysed in the present paper with the methodological principles mentioned above.