This article examines the relationship between health plan performance and physician network overlap.

While health plans are increasingly scrutinized to measure the quality of care provided by participating physicians, many physicians belong to multiple health plans, making it difficult to differentiate between the quality of the health plan and the quality of physician practice. While health plans can influence physician practice patterns, this influence may decrease when physicians accept multiple health plans in their office.

The authors conducted two linear regression models using data from four sources: The National Committee for Quality Assurance, InterStudy, the Dartmouth Atlas, and a private firm that collected information on physicians and their health plan affiliations. Between 2001 and 2003, the authors collected data from 214 nationally representative health plans and their associated physician networks. The authors then linked these data to measures of health plan performance.

Key Findings:

• When there is substantial overlap in health plans' physician networks, measures of quality tend to converge. Health plans whose participating physicians are contracted with multiple plans are associated with overall lower performance quality.
• Health performance measures are more closely associated with physician practice patterns than with health plan efforts to improve performance.

When physicians contract with multiple health plans, the associated health plans have lower measures of performance. This finding may be due to health plans not investing heavily in health quality improvement when they share physicians with many other competing plans. This research also suggests that health plan performance measures may not reflect overall health care quality.