any effect of therapy. Because these other features also influence the clinician's decision to offer the treatment at issue, nonrandomized studies of efficacy are inevitably limited in their ability to distinguish useful from useless or even harmful therapy. As confirmation of this fact, it turns out that studies in which treatment is allocated by any method other than randomization tend to show larger (and frequently "false-positive") treatment effects than do randomized trials [10] [11] [12] [13] . The beauty of randomization is that it assures, if sample size is sufficiently large, that both known and unknown determinants of outcome are evenly distributed between treatment and control groups. What can the clinician do if no one has done a randomized trial of the therapeutic question she faces? She still has to make a treatment decision, and so must rely on weaker studies. In a later article in this series devoted to deciding whether a therapy or an exposure causes harm (a situation when randomization is usually not possible) we deal with how to assess weaker study designs. For now, you should bear in mind that non-randomized studies provide much weaker evidence than do randomized trials. 2. Were all patients who entered the trial properly accounted for and attributed at its conclusion? This guide has two components: was followup complete? and, were patients analyzed in the groups to which they were randomized? a) Was followup complete? Every patient who entered the trial should be accounted for at its conclusion. If this is