title: Consultants' forum: should post hoc sample size calculations be done?
abstract: Pre-study sample size calculations for clinical trial research protocols are now mandatory. When an investigator is designing a study to compare the outcomes of an intervention, an essential step is the calculation of sample sizes that will allow a reasonable chance (power) of detecting a pre-determined difference (effect size) in the outcome variable, at a given level of statistical significance. Frequently studies will recruit fewer patients than the initial pre-study sample size calculation suggested. Investigators are faced with the fact that their study may be inadequately powered to detect the pre-specified treatment effect and the statistical analysis of the collected outcome data may or may not report a statistically significant result. If the data produces a "non-statistically significant result" then investigators are frequently tempted to ask the question "Given the actual final study size, what is the power of the study, now, to detect a treatment effect or difference?" The aim of this article is to debate whether or not it is desirable to answer this question and to undertake a power calculation, after the data have been collected and analysed.
authors.affiliation: Medical Statistics Group, School of Health and Related Research, University of Sheffield, Sheffield, UK. s.j.walters@sheffield.ac.uk
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Walters
mesh_terms: D002986:Clinical Trials as Topic / Q000379:methods*; D006801:Humans; D012107:Research Design; D018401:Sample Size; D013223:Statistics as Topic; D013997:Time Factors; D016896:Treatment Outcome