7. Project Summary/Abstract Statistics fundamentally transformed the practice of and meaning of medicine in the twentieth century. Though insurers and public health reformers had relied on actuarial and statistical calculations for years, formal statistical tests and measures were rarely found in the medical literature in the 1930s. Just a half century later, however, statistical calculations had become essential for clinicians and patients seeking to measure the efficacy of new drugs, choose among treatment options, establish clinical practice guidelines, recommend screening tests, or assess the carcinogenicity of everyday exposures. Statistically-driven innovations such as randomized clinical trials, meta-analyses, n-of-1 and adaptive trials, as well as prospective and retrospective studies have become central tools of modern medicine. Yet we know surprisingly little about the emergence of biostatistics and the related field of clinical epidemiology. Most existing histories of medicine have noted the rise of statistically-interpreted trials as a strategy of physician-reformers looking to reduce the role of bias and subjectivity in medicine but historians have yet to seriously engage with the technical transformations that enabled what was once a field of averages and aggregation (and mainly of use to epidemiologists) to be applicable to decision making at an individual (clinical) level. This transformation was controversial, and faced resistance from many quarters, but particularly from surgical and other procedure-heavy specialties. This grant will support production of a monograph, Number Doctors: The Emergence of Biostatistics and the Reformation of Modern Medicine, that will reframe our understanding of how statistics and statisticians came to play a transformative role in modern medicine. The monograph will partially center on a group of biostatisticians hired in 1947 under sociologist Harold Dorn at the National Cancer Institute, but that soon worked across the National Institutes of Health and other federal agencies, including the Food and Drug Administration, Environmental Protection Agency, and Heath, Education, and Welfare. Other chapters will trace the spread of statistical measures from population genetics and agriculture to biostatistics and clinical epidemiology programs at universities as well as the development of specialized journals and professional organizations. The final chapters will trace the origins of now- common tools like metanalysis and simulation modeling. Drawing on archival materials as well as accounts of medical reformers, the resulting monograph will be the first extended history of biostatistics and clinical epidemiology, and will contribute to larger discussions about the role of data, risk calculations, and the meaning of “evidence-based” and “precision” medicine.