Nearly two-thirds of US office-based physicians work in practices of fewer than seven physicians. It is often assumed that larger practices provide better care, although there is little evidence for or against this assumption. What is the relationship between practice sizeand other practice characteristics, such as ownership or use of medical home processes-and the quality of care? We conducted a national survey of 1,045 primary care-based practices with nineteen or fewer physicians to determine practice characteristics. We used Medicare data to calculate practices' rate of potentially preventable hospital admissions (ambulatory care-sensitive admissions). Compared to practices with 10-19 physicians, practices with 1-2 physicians had 33 percent fewer preventable admissions, and practices with 3-9 physicians had 27 percent fewer. Physician-owned practices had fewer preventable admissions than hospital-owned practices. In an era when health care reform appears to be driving physicians into larger organizations, it is important to measure the comparative performance of practices of all sizes, to learn more about how small practices provide patient care, and to learn more about the types of organizational structures-such as independent practice associations-that may make it possible for small practices to share resources that are useful for improving the quality of care.

Physician networks have a relationship with ACSAs that is independent of the physicians in the network. Physician networks could be an important focus for understanding variations in medical care and for intervening to improve care.

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The timing of the financial incentives in HVBP was not associated with improved quality of care. It is unclear whether improvement for the clinical process measures prior to the start of HVBP was driven by the expectation of the program or was the result of other factors.

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