Fundamentals of Statistics

In order to have confidence that the trial results are representative, it is critically important to have a ‘sufficient’ number of randomly-selected participants in each trial group. Sample size is the total number of participants included in a trial.

An estimate of the required sample size is needed and must be specified in the study protocol before recruitment starts. It is also necessary to control the probability with which a true effect can be identified as statistically significant.

Too few participants or observations will mean that real effects might not be detected, or they will be detected but at a level that is statistically insignificant (a type II error, which is directly proportional to sample size). It is just as true that it is unacceptable for a medicine to be tested on too many patients. Thus, studies with either too few or too many patients are both methodologically and ethically unjustified. Bear in mind that it is only possible to detect rare side effects in a large sample size and sufficient exposure time, this makes the challenging of estimating sample size complex.

In practice, the sample size used in a study is determined based on: