This is a classic article about the relationship between signal, noise, sample size, and confidence. It provided pragmatic guidance on designing a trial to make the best use of limited resources. It should be the first article that you read before you sit down and design a clinical trial.
Sackett DL. Why randomized controlled trials fail but needn’t: 2. Failure to employ physiological statistics, or the only formula a clinician-trialist is ever likely to need (or understand!). CMAJ. 2001;165(9):1226–1237.
You can find an earlier version of this page on my blog.