That should be the simple conclusion from this new Gallup data. They found that there was a noticeable drop in the uninsured in states that took part in the Medicaid expansion as part of the Affordable Care Act –  but a much slower drop in the states that did not. From Gallup:

Changes in States' Uninsured Rates Between 2013 and Quarter 1, 2014

Looking at the enrollment data for the individual states the impact of whether or not a state decided to build their own exchange is probably very minor. Almost all the difference can likely attributed to whether or not a state took advantage of the Supreme Court decision making the Medicaid expansion optional.

This shouldn’t come as a surprised to anyone. If you want the uninsured to have health insurance, the most direct and efficient way to do that is just to have the government automatically give them insurance. That is why Medicaid expansion has always been one of the best aspects of the law.