Solid supporting data for the model’s impact on health costs and treatment outcomes – particularly in peer-reviewed journals – have been scarce, hobbling the ability of these practices to convince larger employers to enroll their workers, says Carolyn Engelhard, who directs the health policy program at the UVA School of Medicine. “Employers don’t want to put out $50 more a month per employee when they are already paying for their health insurance, unless they feel like it’s worth the investment,” she says.