Statistics and ethics in medical research: III How large a sample?

Whatw~r type of statistical design is used for a study, the problem of sample size must be faced. This aspect, which causes considerable difficulty for researchers, is perhaps the most common reason for consulting a statistician. There are also, however, many who give little thought to sample size, choosing the most convenient number (20, 50, 100, etc) or time period (one month, one year, etc) for their study. They, and those who approve such studies, should realise that there are important statistical and ethical implications in the choice of sample size for a study. A study with an overlarge sample may be deemed unethical through the unnecessary involvement of extra subjects and the correspondingly increased costs. Such studies are probably rare. On the other hand, a study with a sample that is too small will be unable to detect clinically important effects. Such a study may thus be scientifically useless, and hence unethical in its use of subjects and other resources. Studies that are too small