The United States purports to have an unusually strong commitment to equal opportunity, yet surprisingly it hasn't collected the mobility data needed to reliably monitor whether that commitment is being upheld. Although mobility and opportunity cannot of course be equated, it's widely understood that mobility data provide fundamental evidence on opportunity, which is why virtually all late industrial countries, save the U.S., have well-developed systems for monitoring mobility. It's not as if the U.S. is a more general laggard in developing social indicators.