As Gallup previously reported, the states that have chosen to expand Medicaid and set up their own healthcare exchanges had a lower average uninsured rate to begin with: 16.1% compared with 18.7% for the remaining states — a difference of 2.6 points. The already notable gap between the two groups of states widened in the first quarter to 4.3 points.

Nationally, 17.3% of U.S. adults reported being without health insurance in 2013, a rate that had slowly increased from 14.8% in 2008. The uninsured rate peaked at 18.0% in the third quarter of 2013 — the three months immediately preceding the opening of the healthcare exchanges — and has since declined to 15.6%.

These data, collected as part of the Gallup-Healthways Well-Being Index, are based on Americans’ self-reported insurance status in response to the question, “Do you have health insurance coverage?”