Obama’s Medicare head is resigning. Here’s the most important thing she did in office.

Centers for Medicare and Medicaid Services administrator Marilyn Tavenner will leave the agency at the end of February, according to an email sent to agency staff early Friday morning.

CMS is a big, hulking agency that controls a sizable chunk of the federal budget. It runs two public health insurance programs, Medicare and Medicaid, that collectively insure one in three Americans. These are health insurance plans that are big enough to catalyze real change in the health care system. If Medicare, for example, decides to reimburse doctors for a certain procedure, private insurers will typically follow.

During her three-year tenure as Medicare head (two years spent as acting administrator before her Senate confirmation), Tavenner mostly has a reputation as a pragmatic and hard-working bureaucrat. The former nurse was known as a leader who cared more for running a government agency effectively than for musing on big, sweeping health reform ideas. (My favorite story about Tavenner, however, is one from her nursing days when she helped bring a patient back from the dead. Seriously.)

Her work at the agency, however, does seem to show a particular agenda for making health care in America better. If Tavenner has a legacy in the federal government, ProPublica’s Charlie Ornstein rightly points out that it will be one of data liberation.

During her two years as Medicare’s head, she’s presided over some of the agency’s more sweeping data releases. Some of this has to do with mandates in the Affordable Care Act, like disclosing the payments that doctors get from pharmaceutical companies.

But some of it wasn’t.

Arguably the biggest data dump of the Tavenner era was the 2013 release of the prices that every American hospital charges for the most common medical procedures. This data revealed huge disparities in health care charges: on the same street in Miami, for example,the data showed one hospital charging $66,030 for inserting a pacemaker — and another with a $127,038 sticker price.

The federal government didn’t have to release this data, but, under Tavenner’s leadership it did. Her agency decided to shine a light on the huge differences in what systems charge for medicine, and the health care system is arguably better for it.