Understanding clinical trials

Abstract

Randomised trials provide not only qualitative conclusions about whether a treatment is better but also quantitative estimates of the extent to which it is better. Take as an example (with rounded numbers) an overview of the effects of treatment with antihypertensive drugs on patients with moderate to severe hypertension. Trials have shown that about 1800 strokes occur among 15 000 patients randomised to active drugs for an average of five years, while about 3000 such events occur in 15 000 randomised to placebo or no active treatment.2 This difference in stroke is extremely unlikely to be due to chance (in other words it is highly significant statistically), providing the qualitative answer that antihypertensive drugs are better than no treatment for patients with moderate to severe hypertension. These results also tell us a lot about how much better drug treatment is, but the way in which this quantitative difference is expressed can lead clinicians to different conclusions about whom to treat - whether …