Measurements of health care spending and outcomes in a geographic area and comparisons of one area to another have been used to make observations about health delivery systems and guide health care policy. Medicare claims files are a ready source of data about health care utilization and have served as the basis for a large number of studies in the United States. If ecologic studies are to accurately reflect local practices, potential variables must be accounted for. In the United States, differences in disease burden and socioeconomic factors are important variables affecting health care spending and outcomes. The assertion that regional variation in Medicare spending in the last two years of life is indicative of widespread waste in the U.S. health care system became a controversial part of the health care reform debate in 2009-2010.