for efficacy. Not so. The reasons people don't take their medication are often related to prognosis. In a number of randomized trials non-compliant patients have fared worse than those who took their medication as instructed, even after taking into account all known prognostic factors, and even when their medications were placebos! [14] [15] [16] [17] [18] [19] Excluding non-compliant patients from the analysis leaves behind those who may be destined to have a better outcome and destroys the unbiased comparison provided by randomization. The situation is similar with surgical therapies. Some patients randomized to surgery never have the operation because they are too sick, or suffer the outcome of interest (such as stroke or myocardial infarction) before they get to the operating room. If investigators include such patients, who are destined to do badly, in the control arm but not in the surgical arm of a trial, even a useless surgical therapy will appear to be effective. However, the apparent effectiveness of surgery will come not from a benefit to those who have surgery, but the systematic exclusion of those with the poorest prognosis from the surgical group. This principle of attributing all patients to the group to which they were randomized results in an "intention-to-treat" analysis. This strategy preserves the value of randomization: prognostic factors that we know about, and those we don't know about, will be, on average, equally distributed in the two groups, and the effect we see will be just that due to the treatment assigned.