People who can go to the emergency room for free do it more than those who might have to pay for it, according to a newly released study based in Oregon.

That’s hardly surprising, on the face of it, given that it fits with the basic economic principle that people use more of something when it costs them less.

But it is at odds with one of the notions underpinning the Affordable Care Act: that expanding Medicaid coverage would reduce expensive emergency room use because patients would seek less costly treatment in primary care settings.

The study, published in the journal Science, took advantage of an unusual opportunity presented by a 2008 Medicaid expansion in Oregon, in which a lottery identified 30,000 people for coverage out of a 90,000 pool of applicants. Researchers compared emergency room use among those who got the coverage and those who didn’t. It was a rare opportunity for a randomized-controlled study of the causal effect of coverage.

Over an 18-month period, researchers found those with Medicaid coverage used emergency care 40 percent more than those without, including visits for problems that could be addressed in a primary care setting. Some experts suggested patients relied on old habits of emergency room use, and their approach might change over a longer time frame, according to The New York Times.

That type of change is the focus of a separate experiment now underway in Oregon, directed by Coordinated Care Organizations. The CCOs are exploring the effects of bundled payments for Medicaid patients that require providers to coordinate and share compensation. Care coordinators help patients get appropriate care, with the hope of keeping them healthy and preventing unneeded ER trips.

The CCO experiment was not yet in effect during the time of this study on emergency room use, and it appears focused on the kind of education that could make a difference. A careful evaluation of its effectiveness will help clarify the meaning of this new evidence about emergency room use.

Only by focusing on solid evidence, rather than ideology, will we be able to navigate the complexities of this massive experiment on health care.