The results of clinical trials are often expressed in relative terms - for example, a particular treatment reduces the risk of an adverse outcome by 40%. Yet knowing that the treatment reduces the risk of such an outcome from 5% to 3% (an absolute reduction of 2%) may be more useful clinically.1

The effects of antiplatelet treatment are a case in point. The meta-analysis of randomised controlled trials of such treatments by the Antiplatelet Trialists' Collaboration shows that the risk of dying from a vascular cause varies substantially according to patient group (p 81).2 It ranges from 10% over one month for patients with an acute myocardial infarction at entry to the trial, through 9% over two years for those who had survived an acute myocardial infarction, down to only 2% over five years in trials of primary prevention.

With such varying risks of future vascular events it is unsurprising that the absolute effects of antiplatelet agents differ substantially by patient group. Reductions in relative risks may be similar - antiplatelet drugs reduce the risk of future non-fatal myocardial infarction by 30% in trials of both primary and secondary prevention. But when the results are presented as the number of patients who need to be treated for one non-fatal myocardial infarction to be avoided they look very different.

In secondary prevention 50 patients need to be treated for two years, while in primary prevention 200 patients need to be treated for five years, for one non-fatal myocardial infarction to be prevented. In other words, it takes 100 patient-years of treatment in secondary prevention or 1000 patient-years of treatment in primary prevention to produce the same beneficial outcome of one …