A formal sample size calculation, justifying the proposed study size and demonstrating that the study is capable of answering the questions posed, is now a component of a research proposal required by most funding agencies. Too often, medical research studies have been too small, because the sample size was decided on purely logistic grounds, or by guesswork. This is not only bad practice: it is considered by many to be unethical because of the waste of time and potential risk to patients participating in a study that cannot answer its stated research question. On the other hand, studying many more persons than necessary is also a waste of time and resources. In a clinical trial, conducting a study that is too large may also be unethical, because this could mean that more persons than necessary were given the placebo, and that the introduction of a beneficial therapy was delayed. Calculating the required sample size requires that we quantify the objectives of our study.