Of course, the increases in states that have expanded Medicaid are more extreme. In Kentucky, the state with the biggest increase, the Medicaid rolls have grown by 71 percent. Overall, states that expanded Medicaid saw substantially larger reductions in the number of people without health insurance. If every state had expanded, we could expect many more people to be insured through the program. (Using data from the advocacy group Enroll America and the data firm Civis Analytics, my colleague Kevin Quealy created a map of what the country would look like if that had happened.)

This week, the Centers for Disease Control and Prevention published the results of a detailed survey of American households this year. It found a 3.4 percentage point reduction in the number of adults under 65 lacking health insurance between 2013 and the second quarter of this year. Its findings are not a surprise — earlier, private polls had shown similar drops. But because of its sample size and methods, the CDC study is the most reliable sign yet that millions more Americans have gotten health insurance this year.

The Medicaid enrollment increases in these nonexpansion states show us one of the ways that the federal health law has led to those reductions.