The “Medicaid undercount” refers to the discrepancy between administrative counts of Medicaid enrollment and estimates from survey data. Nearly all state and federal surveys estimate fewer Medicaid enrollees than is described by enrollment records. In the 2005 Current Population Survey (CPS) 40.8% of people known to have Medicaid from administrative records were not reported as having Medicaid in the CPS (SNACC Phase V,2010). In the 2002 National Health Interview Survey (NHIS) the undercount was 33.5% (SNACC Phase IV, 2009) and in the 2003 Medical Expenditure Panel Survey Household Component (MEPS/HC) it was 17.5% (SNACC Phase VI, 2010). State sponsored surveys often fare better, but still tend to undercount Medicaid by 12-26% (Call, Davern, Klerman & Lynch, 2013). This mismatch suggests that survey data, relative to what is known from administrative data, give biased estimates of key policy measures such as the share of the population covered by Medicaid or those lacking health insurance. Although survey data on Medicaid is likelybiased, unlike administrative data, surveys provide a wide array of policy relevant covariates such as access to health services, health status, and race and ethnicity, and surveys are the only source of information about the uninsured and the eligible but not enrolled.