In this paper we discuss the appropriate application of inferential statistics to practice profiles and other measures of care. To accomplish our objectives, we first describe the relative merits of measuring three well-recognized domains of medical quality: structure, process, and outcome. Next, we discuss inferential statistics as used in quality improvement. We then describe several common circumstances that arise in the measurement of medical care, giving attention to the application of inferential statistics to each situation. We end with a brief discussion of statistical techniques commonly used in the measurement of quality and challenges that arise with their use.