The TDI Data Analytic Core and the CMS Medicare Data at Dartmouth

Three decades of advancing health services research
1996 - 2026

The Dartmouth Institute for Health Policy and Clinical Practice (TDI)
Geisel School of Medicine at Dartmouth

---

Introduction  

This document serves as a memorialization of the TDI Data Analytic Core (DAC) and the Centers for Medicare & Medicaid Services (CMS) data enterprise at Dartmouth College. For more than three decades, these resources stood at the heart of a research program that fundamentally transformed the way the United States understands geographic variation in health care delivery, spending, and outcomes. With the discontinuation of the DAC on June 30, 2024, and changes to CMS data access policies, we record here the origins, achievements, and enduring impact of this work.

I. Origins: A Discovery in Vermont  

In 1973, Dr. John E. “Jack” Wennberg and Alan Gittelsohn published a landmark article in Science magazine titled “Small Area Variations in Health Care Delivery.” This paper documented what no one had previously demonstrated at scale: that the health care patients received in the United States varied dramatically based on where they lived—and that these variations bore little relationship to the actual health needs of the population.

Studying the small state of Vermont, Wennberg expected to find widespread underservice. Instead, he discovered enormous variation in hospital admissions, surgical procedures, health care resource allocation, and per-capita expenditures across hospital service areas. These differences could not be explained by patient demographics or illness burden. Rather, they appeared to reflect professional variation—physicians in different regions simply practiced medicine differently, in the absence of clinical evidence to guide them.

Not a single medical journal would publish the original paper. It was Science that gave it a home. For years, the work was largely ignored. But Wennberg persisted, and his findings would eventually reshape American health policy.

II. Building the Institutional Foundation  

In 1988, Wennberg founded the Center for the Evaluative Clinical Sciences at the Dartmouth Medical School—what would later become The Dartmouth Institute for Health Policy and Clinical Practice (TDI) at the Geisel School of Medicine . This center was purpose-built to address unwarranted variation in health care and to develop the analytical infrastructure needed to study it on a national scale.

The election of President Bill Clinton in 1992 and the expectation of national health care reform provided the impetus for a more ambitious undertaking. Wennberg’s team recognized the need for accurate, objective data that could demonstrate the impact of legislation on the entire U.S. health care system. National Medicare datasets, analyzed using the small area methods developed during the 1970s and 1980s, were the ideal instruments.

With the support of the Robert Wood Johnson Foundation, work began to define health care market areas for the nation, resulting in the creation of Hospital Referral Regions (HRRs) and Hospital Service Areas (HSAs) that would become the geographic backbone of all subsequent Dartmouth Atlas analysis.

III. The Dartmouth Atlas of Health Care (1996–2024)  

Though the Clinton health plan ultimately failed to pass, the first Dartmouth Atlas of Health Care was published in 1996. The national edition focused on data at the Hospital Referral Region (HRR) level and was accompanied by nine regional volumes reporting local data for Hospital Service Areas (HSAs). This Atlas brought together and expanded upon the themes of Wennberg’s decades of research into a single, publicly accessible resource.

Over the next twenty-five years, the Dartmouth Atlas Project produced more than forty reports addressing a wide range of health care problems—from end-of-life care and surgical utilization to hospital readmissions, chronic illness management, and Medicare spending patterns. The project used CMS Medicare and Medicaid claims data to provide analysis at national, regional, and local levels, as well as at the hospital and physician-affiliation level.

The Atlas’s influence was profound. Between 1996 and 2023, more than 8,700 peer-reviewed articles referenced the Dartmouth Atlas or used its data and tools. Multiple provisions of the Affordable Care Act (ACA) of 2010 can be traced directly to findings from the Atlas, including demonstration projects and reforms targeting unwarranted variation in Medicare spending.

IV. The Data Analytic Core  

The Data Analytic Core (DAC) served as a federally regulated computing service center that made the Dartmouth Atlas and its associated research possible. The DAC managed the storage and analytic needs of TDI’s repository of Medicare and Medicaid data, operating under strict CMS Data Use Agreements (DUAs) that governed every aspect of data access, security, and dissemination.

Data Holdings  

The DAC maintained an extensive collection of CMS research files hosted on secure servers known as “Granite.” These files included 100% of beneficiary demographic and enrollment information (Master Beneficiary Summary File) and 100% claims for inpatient hospital utilization (Medpar), physician services (Part B/Carrier), Outpatient services, Home Health Agency services, Durable Medical Equipment services, Hospice services, and 40% Part D prescription drug files. In addition to the Medicare data, the DAC also hosted data such as HEDIS quality measures, census data, and American Hospital Association data.

Security and Compliance  

CMS required strict protocols for handling protected health information. All data movement on or off the DAC’s information systems required formal data transfer requests, with only authorized Dartmouth Information, Technology & Consulting (ITC) /Research Computing and Data (RCD) personnel permitted to execute transfers. The DAC enforced CMS cell-size suppression policies, requiring that no reported cell contain a value of 1 to 10 and that no cell allow such a value to be derived. Users who were flagged for suppression errors three times in a year had their Granite access revoked pending additional training.

The Atlas Rate Generator  

Among the DAC’s contributions was the Atlas Rate Generator, a tool that allowed researchers to efficiently obtain cross-sectional rates using Medicare data. The Rate Generator lowered a common barrier to research by enabling investigators to run preliminary analyses during the grant development phase, avoiding the costly and time-consuming process of applying for and building individual databases.

Coding Trends and Data Quality  

Each year, the DAC generated and published Coding Trends reports derived from Medicare MedPAR, Outpatient, and Carrier (Part B) files. These reports tracked year-to-year counts of diagnosis and procedure codes—including ICD-9/ICD-10 codes, HCPCS codes, revenue center codes, DRGs, and MSDRGs—providing researchers with essential tools for monitoring data quality and identifying coding shifts over time.

V. Policy Impact and Legacy  

The work enabled by the DAC and CMS data at Dartmouth left a deep imprint on American health policy. Key milestones include:

Section 646 of the Medicare Prescription Drug, Improvement, and Modernization Act of 2003 authorized CMS to create “Comprehensive Centers for Medical Excellence,” explicitly grounded in Wennberg’s body of research.

The Affordable Care Act of 2010 incorporated multiple provisions addressing geographic variation in spending and quality, informed directly by Dartmouth Atlas findings.

Over 8,700 peer-reviewed publications referenced or drew upon Atlas data and methods between 1996 and 2023, establishing small area variation analysis as a cornerstone of health services research.

VI. The End of an Era  

Two converging events brought this chapter of Dartmouth’s history to a close. Dr. Jack Wennberg, the founder of the Dartmouth Atlas and the intellectual architect of small area variation research, passed away on March 10, 2024, at the age of 89. Concurrently, CMS adopted a policy phasing out on-premises access to research data, making the DAC’s operating model untenable.

The Data Analytic Core was formally discontinued on June 30, 2024, but remained in operation until May 5, 2026. Dartmouth’s Information, Technology, and Consulting (ITC) group, with support from Research Computing (RC), assumed responsibility for several of the services previously provided by the DAC. The Dartmouth Atlas website continues to provide historical rates through 2019 and associated documentation , but no new annual rates will be calculated.

In Appreciation  

We honor the vision of Dr. Jack Wennberg, the dedication of the researchers, analysts, and staff who built and sustained this work, and the commitment of The Dartmouth Institute and the Robert Wood Johnson Foundation. Their collective effort transformed a single observation about a small New England state into a body of evidence that reshaped how a nation understands and manages its health care system.

Although the TDI DAC has closed and the Atlas will no longer be updated, the legacy endures—in the thousands of publications it inspired, the policies it shaped, and the new generation of Dartmouth researchers who carry its mission forward.