Improving the U.S. health-care system requires encouraging low-value doctors and hospitals to practice as well as high-value ones do. The gap between the two is wide, but that only shows how much room we have for improvement. Costs vary wildly across regions, among hospitals within a region, and even among doctors within a given hospital. Because this variation doesn't appear to be reliably correlated with differences in quality, value seems to be much higher in some settings than in others. What is causing this, and what might we do about it?