Factors influencing the sample size, exemplified by studies on gastroduodenal tolerability of drugs.

The necessary number of subjects to be included in a clinical trial depends on different factors. Exemplified by clinical trials on gastroduodenal tolerability of NSAID, we show how the sample size increases when the significance level decreases, the detection level increases, or the clinically relevant difference decreases. The sample size also increases when the trial design is changed from a cross-over to a parallel-group design. It is concluded from an ethical and statistical point of view that the sample size must be calculated when planning a study.