How is managed care activity currently being measured, and are these measures accurately capturing changes in the market? Stanford University researchers compiled and assessed measures of managed care activity in order to examine: 1) how well existing measures of managed care activity reflect the variation that actually exists in managed care organizations; and 2) how these measures compare with each other. In addition, they conducted a small number of example analyses that used different measurement tools (including the Community Tracking Study Household Survey) to illustrate how using different measures for the same analysis effects the results. They analyzed data from the Community Tracking Study Household Survey data as well as measures of HMO activity at the MSA level constructed from those data, InterStudy survey data, HCFA Medicare HMO data, SMG Marketing PPO data, the AMA survey on physician revenues from managed care, and Florida and California state hospital data on hospital expected source of payment. Their objective was to identify strengths and weaknesses of each measure evaluated, and test the hypothesis that while many researchers are using readily available measures of managed care activity interchangeably, the measures are actually quite different and produce different results.