A study published in the Journal of the American Medical Association
found that 65% of primary-care docs work in practices that are too small to draw meaningful conclusions about the quality of care they provide. Researchers looked at commonly cited quality measures such as blood-sugar control in diabetics and mammogram screening for women. Their cutoff was the ability to reliably discern when a doctors in a practice varied by 10% from the national average. They found that "only the largest primary care physician practices, which are also the most uncommon, can be expected to have sufficient caseloads to measure significant differences in performance."