Fisher’s randomization rule has been widely viewed as a revolutionary invention in experimental design. The three rationales of randomization in clinical trials are (i) randomization ensures that known and unknown confounders are asymptotically controlled, (ii) the use of randomization itself provides the basis of statistical inference, supposing patients in a clinical trial are a non-random sample of a population, and (iii) the act of randomization mitigates selection bias by providing unpredictability in treatment allocation. Randomized controlled trials have been the gold standard for more than five decades, while such trials may be costly, inconvenient and ethically challenging. Some Fisherian statisticians have emphasized the importance of design-based inference based on randomization test, however some statisticians does not agree with them. From the Bayesian point of view, the randomization sequence is ancillary for a parameter of interest, and randomization itself is not absolutely essential although it may sometimes be helpful. In this review, I provide an overview of the rationales of randomization and the related topics, and discuss the significance and limitations of randomization in clinical trials.
