Whenever an epidemiological study is being planned, the question always arises of how many subjects to include, that is, what sample size to use. This is clearly a vital question and would constitute a crucial part of any research protocol. Too large a sample means wasted resources. On the other hand, too small a sample leads to lack of precision in the results. Below are some general formulas used to calculate sample sizes.
when testing a difference between means
when testing a proportion
when testing a relative risk
when testing case-control studies