Since its inception in World War II, the clinical trial has evolved into a standard procedure in determining therapeutic efficacy in many Western industrial democracies. Its features include a control group of patients that do not receive the experimental treatment, the random allocation of patients to either the experimental or control group, and the use of blind assessment so that researchers do not know which patients are in which group. Even though the clinical trial is a relatively recent development, comparative statistics in a therapeutic context have a much longer history. From that history, this book chooses to discuss three crucial debates: that among clinicians before the Parisian Academy of Medicine in 1837, the debate in the German physiological literature during the 1850s, and, in the early 20th century, the debate over the bacteriologists' diagnostic technique involving the opsonic index. Matthews demonstrates that despite the very real differences separating clinician, physiologist and bacteriologist, they all shared an antipathy toward the methods of the statistician. Since they viewed medical judgment as a form of tacit knowledge, they downplayed the concerns of the medical statistician who was attempting to make medical inference into something explicit and quantitative. Only when medical decision-making moved from the cloistered confines of professional medical expertise into the arena of open political debate could the medical statistician (and the clinical trial) gain the upper hand.