Medicaid expansion and the Medicaid undercount in the American Community Survey

To measure discordance between aggregate estimates of means-tested coverage from the American Community Survey (ACS) and administrative counts and examine the association of discordance with ACA Medicaid expansion.

2010-2016 ACS and counts of Medicaid and Children's Health Insurance Program enrollment from the Centers for Medicare & Medicaid Services.

State-by-year counts of means-tested coverage from the ACS were compared to administrative counts using percentage differences. Discordance was compared for states that did and did not adopt expansion using difference-in-differences. We then contrasted the effect of expansion on means-tested coverage estimated from the ACS with results from administrative data.

Survey and administrative data.

One year before expansion there was a 0.8 and 4 percent overcount in expansion and nonexpansion states, respectively. By 2016, there was a 10.64 percent undercount in expansion states vs a 0.02 percent undercount in nonexpansion states. The ACS suggests that expansion increased means-tested coverage in the full population by three percentage points, relative to five percentage points suggested by administrative records.

Discordance between the ACS and administrative records has increased over time. The ACS underestimates the impact of Medicaid expansion, relative to administrative counts.