Study Examines Variation, Factors Involved With Patient-Sharing Networks Among Physicians in U.S.


Media Advisory: To contact Bruce E. Landon, M.D., M.B.A., call David Cameron at 617-432-0441 or email To contact editorial co-author Elliott S. Fisher, M.D., M.P.H., call Annmarie Christensen at 603-653-0897 or email

CHICAGO – Physicians tend to share patients with colleagues who have similar personal traits and practice styles, and there is substantial variation in physician network characteristics across geographic areas, according to a study in the July 18 issue of JAMA.

Physicians are embedded in informal networks that result from their sharing of patients, information, and behaviors. “These informal information-sharing networks of physicians differ from formal organizational structures (such as a physician group associated with a health plan, hospital, or independent practice association) in that they do not necessarily conform to the boundaries established by formal structures,” according to background information in the article. “The potential influence of informal networks of physicians on decision making has been understudied despite the potential importance of these networks in day-to-day practice.”

Bruce E. Landon, M.D., M.B.A., of Harvard Medical School, Boston, and colleagues conducted a study to identify professional networks among physicians, examine how such networks vary across geographic regions, and determine factors associated with physician connections. Using methods adopted from social network analysis, Medicare administrative data from 2006 were used to study 4,586,044 Medicare beneficiaries seen by 68,288 physicians practicing in 51 hospital referral regions (HRRs). Distinct networks depicting connections between physicians (defined based on shared patients) were constructed for each of the 51 HRRs. The randomly sampled HRRs are distributed across all regions of the country and include urban and rural locations. The average physician age was 49 years and about 80 percent were male. Among the Medicare patients, the average age was 71 years and 40 percent were male.

Substantial variation was observed across HRRs. The number of included physicians ranged from 135 in Minot, N.D., (1,568 ties) to 8,197 in Boston (392,582 ties). The average adjusted degree (number of other physicians each physician was connected to per 100 Medicare beneficiaries) across all HRRs was 27.3. Patterns varied by physician specialty as well. Physicians with ties to each other were far more likely to be based at the same hospital (69.2 percent of unconnected physician pairs vs. 96.0 percent of connected physician pairs). Connected physician pairs also were more likely to be in close geographic proximity. The average distance for connected pairs was 13.1 miles vs. 24 miles.

“Characteristics of physicians’ patient populations also were associated with the presence of ties between physicians.  Across all racial and ethnic groups, connected physicians had more similar racial compositions of their patient panels than unconnected physicians. For instance, connected physician pairs had an average difference of 8.8 points in the percentage of black patients in their 2 patient panels compared with a difference of 14.0 percentage points for unconnected physician pairs. Similarly, differences in mean [average] patient age and percentage of Medicaid patients also were smaller for connected physicians than unconnected physicians. Medical comorbidities [co-existing illnesses] were also more similar, suggesting that connected physicians had more similar patients in terms of clinical complexity than unconnected physicians,” the authors write. “Physicians thus tend to cluster together along attributes that characterize their own backgrounds and the clinical circumstances of their patients.”

“It has long been known that physician behavior varies across geographic areas, yet our understanding of the factors that contribute to these geographic differences is incomplete. Our findings suggest that variation according to network attribute might help explain health care variation across geographic areas, particularly given what is known about how networks function.”

“In conclusion, we used novel methods to define social networks among physicians in geographic areas based or shared patients, examined how such networks vary across different geographic regions, and identified physician and patient population factors that are associated with physician patient-sharing relationships. This approach might have useful applications for policy makers seeking to influence physician behavior (whether related to accountable care organizations or innovation adoptions) because it is likely that physicians are strongly influenced by their network of relationships with other physicians,” the researchers write.

(JAMA. 2012;308[3]:265-273. Available pre-embargo to the media at

Editor’s Note: This work was supported by a grant from the National Institute on Aging. Dr. Barnett was supported by a Doris Duke Charitable Foundation Clinical Research Fellowship and a Harvard Medical School Research Fellowship. Please see the article for additional information, including other authors, author contributions and affiliations, financial disclosures, etc.

Editorial: Social Networks in Health Care – So Much to Learn

In an accompanying editorial, Valerie A. Lewis, Ph.D., and Elliott S. Fisher, M.D., M.P.H., of Dartmouth Medical School, Hanover, N.H., discuss the importance of networks in health care.

“As advances in biomedicine enhance the effectiveness and complexity of the medical interventions required to treat acute and chronic illness, it is becoming clear that meeting physicians’ professional responsibility to each patient will require new ways of working together. Evidence from fields outside medicine, summarized in Putnam’s ‘Bowling Alone,’ concludes that stronger connections within a given group (such as physicians) and across groups (such as between physicians, nurses, and administrators) can help create the trust and shared values that are crucial to organizational success and individual fulfillment. Further research along the lines developed by Landon and colleagues should help bring useful insights to health care.”

(JAMA. 2012;308[3]:294-296. Available pre-embargo to the media at

Editor’s Note: Please see the article for additional information, including financial disclosures, funding and support, etc.

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