application of epidemiological and statistical methods to the study of hospital deaths, and her discoveries of a plausible causal relationship between processes of care and outcomes led to challenges to thoughts about mechanisms of disease prevalent at the time, changes in clinical practice, and improvements in mortality rates. In the early twentieth century, Ernest Amory Codman, a Boston surgeon, argued strongly for the formal study of surgical outcomes in an effort to understand which surgeons, hospitals, and surgical procedures produced good versus bad outcomes (Neuhauser, 2002). This effort did not take root and grow—in fact, it provoked significant hostility among Codman’s colleagues—but it raised the question of the need for formal analysis of treatment outcomes that was picked up again more than 50 years later.

The need for formal evidence of effectiveness for common medical and surgical interventions was recognized much more broadly beginning in the 1970s. Passage of Medicare and Medicaid legislation, together with apparent advances in medical and surgical care, contributed to a surge in health care spending. Policy makers and payers asked increasingly pointed questions about the “value” of health care, questions that required more fundamental questions about the effectiveness of interventions to be answered.

Even more disquieting questions emerged from a body of work that described striking variations in the rates of common surgical procedures such as surgery for benign prostate disease, hysterectomy, and tonsillectomy, among seemingly similar geographic regions. This work began in the late 1960s in northern New England, where isolated hospital market areas could easily be defined (Wennberg and Gittelsohn, 1982).

International differences in the rates of medical procedures were observed; and when the variations within countries were adjusted for the overall rate of variation among countries, a consistent pattern was detected; a high degree of variation was a marker for a high degree of discretion, and a high degree of discretion was often explained by professional uncertainty about effectiveness (McPherson et al., 1982). At the time, few of the procedures in question had been subjected to randomized controlled clinical trials or other credible clinical studies. Subsequently, variations in the rates of medical admission, physician visits, and diagnostic tests that could not be explained by clinical variables were also found. Taken together, these findings raised new questions about the science base of clinical practice. If decisions were based on science, how could it be that treatment depended more on where one lived than what was wrong or what one cared about? Policy makers wondered if high rates meant overuse and economic waste and if low rates meant underuse and deprivation. “Which rate was right?” became the pressing policy question and the answer required a new investment in clinical research to better define the outcomes of common interventions for common conditions. Thus, the practice variation phenomenon

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