Clinical trials are expensive, whether the cost is counted in money or in human suffering, but they are capable of providing results which are extremely valuable, whether the value is measured in drug company profits or successful treatment of future patients. Balancing potential value against actual cost is thus an extremely important and delicate matter and since, other things being equal, both cost and value increase the more patients are recruited, determining the number needed is an important aspect of planning any trial. It is hardly surprising, therefore, that calculating the sample size is regarded as being an important duty of the medical statistician working in drug development.
I’ll pile on the references by linking to chapter 20 of my book with Jennifer, which, compared to Senn’s book, has a bit more calculation and a bit less discussion. I suspect many readers will benefit from reading both. (Full link to the book here.)
Finally, here’s a link with a couple more references, including a great little article by Russ Lenth.