Health insurers assign doctors and hospitals to tiers using a complicated formula of quality and cost measures. In short, the tiers are different because insurers don't use all the same quality measures, because they give the measures different weight and because insurers pay physicians and hospitals different rates. Massachusetts will soon require that health insurers use the same set of quality criteria. But hospital and physician ratings will still vary because insurers will still weigh the criteria differently and because the prices insurers pay for office visits, tests and procedures are not the same.
