The pre-post treatment comparison as an indicator of treatment effect does away with the RCT requirements of a control group, of randomization, and of evaluator blinding. Another unacceptable alternative acknowledges the necessity of a control group but tries to do away with randomization and blinding: the historical control group.
Here, as in pre-post studies, all patients recruited into the study receive T. However, information is obtained from clinical records and from past RCTs and other research studies on what happened to patients who received C—a historical rather than a concurrent control group. The treatment effect is then inferred from comparisons of the T group with the historical control group.
It has long been known that when one recruits patients from different sources (e.g., different sites, different recruitment pools, or at different times), what is seen, even in response to the same treatment in samples satisfying the same inclusion/exclusion criteria, is different. It is usually hoped that the effects of T versus C within each source are much the same (generalizability) in patients recruited from different sources, but the responses within the individual T and C groups are expected to differ sharply in patients recruited from one source versus another. With a historical control group, any difference seen may be due wholly or in part to site/time/context effects rather than being due to the effect of T versus C, and the effect size that is estimated and tested is not valid for establishing the efficacy or effectiveness of a treatment.
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