Compiled by the non-profit, non-partisan Health Care Cost Institute, the database will allow researchers to slice and dice more than 3 billion medical claims for more than 33 million individuals in search of answers. The previously confidential information, scrubbed of identifying details, is being provided by three of the nation's largest insurance companies: Aetna, Humana, and UnitedHealthcare—whose combined customers account for about 20 percent of Americans under age 65 who are insured through an employer.