Researchers from the Urban Institute and the State of Massachusetts explored why existing surveys generate very different estimates of the uninsurance rate in Massachusetts. The surveys they examined are the Current Population Survey (CPS), the Behavioral Risk Factor Surveillance System (BRFSS), the Massachusetts Health Insurance Survey, and the Massachusetts Health Reform Survey (MHRS). This brief described how estimates may vary because of differences in the wording of the insurance questions asked in the surveys, differences in question placement and context within the survey, differences in survey design and fielding strategies, differences in accounting for missing data and other data preparation, and differences in survey fielding time frames. The analysis concludes that there has been no single survey in Massachusetts that is clearly superior across all of these important dimensions.