History of Randomized Controlled Trials The history of clinical trials dates back to

receding articles in this series have provided a great deal of in-formation concerning research design and methodology, including research protocols, statistical analyses, and assess-ment of the clinical importance of radiologic research studies. Many methods of research design have already been presented, includ-ing descriptive studies (e.g., case reports, case series, and cross-sectional surveys), and some analytical designs (e.g., cohort and case-control studies). Case-control and cohort studies are also called observational studies, which distin-guishes them from interventional (experi-mental) studies because the decision to seek one treatment or another, or to be exposed to one risk or another, was made by someone other than the experimenter. Consequently, the researcher’s role is one of observing the outcome of these exposures. By contrast, in experimental studies, the researcher (experi-menter) controls the exposure. The most powerful type of experimental study is the randomized controlled trial. The basic prin-ciples of randomized controlled trials will be discussed in this article