In an unprecedented study of 2.2 million emergency room visits across the United States, Yale researchers found that 22% of patients who went to emergency departments within their health-insurance networks were treated by an out-of-network doctor and potentially incurred major, unexpected expenses. The study, based on data from a large commercial insurer that included tens of millions of covered individuals, provides the first national estimate of the frequency of "surprise" out-of-network medical billing. The researchers describe their findings in an article published in the Nov. 16 edition of the New England Journal of Medicine.