In biostatistics as applied to clinical trial interpretation, a post hoc (after-the-fact) subgroup analysis examines relationships in trial data that were not pre-specified in the study protocol, meaning it is exploratory and hypothesis-generating rather than confirmatory. Because such analyses are not protected against multiple-comparisons error, an apparently significant finding may reflect chance alone (per conventional alpha thresholds, roughly one spurious "significant" result is expected for every twenty comparisons performed), so post hoc findings require independent, prospectively designed confirmatory trials before being treated as evidence of an effect. This principle sits within the broader methodology of prospective, randomized, and appropriately powered and stratified trial design, which is the standard against which retrospective or exploratory analyses are distinguished.
In biostatistics as applied to clinical trial interpretation, a post hoc (after-the-fact) subgroup analysis examines relationships in trial data that were not pre-specified in the study protocol, mea…