Random sampling variation is greater at low volumes,309,797,798,803,827,834,835 which compli- cates performance assessment of smaller programs. Several strategies may be considered to mitigate this measurement issue, including analysis over longer periods of time; appro- priate statistical methodologies, such as hierarchical (random- effects) models; composite measures, which effectively in- crease the number of endpoints; and statistical quality control approaches, such as funnel plots835 and cumulative sum plots.836 838 Small programs may benefit from direct super- vision by a large tertiary center.834 Ultimately, state or national regulatory authorities must decide whether the lower average performance of very small programs and the added difficulty in accurately measuring their performance are outweighed by other considerations, such as the need to maintain cardiac surgery capabilities in rural areas with limited access to referral centers.834 Volume, a structural quality metric, is an imperfect proxy for direct measurement of outcomes.822,839 Risk- adjusted outcomes based on clinical data are the preferred method of assessing CABG quality except in very low- volume programs, in which performance is generally weakest and small sample size makes accurate assessment of performance difficult. 5.2. Adverse Events 5.2.1. Adverse Cerebral Outcomes The incidence of stroke after CABG ranges from 1.4% to 3.8%,840 depending on the patient population and the criteria for diagnosis of stroke. Risk factors for stroke include advanced age, history of stroke, diabetes mellitus, hyperten- sion,841 and female sex,842 with newer research emphasizing the importance of preoperative atherosclerotic disease (in- cluding radiographic evidence of previous stroke or aortic atheromatous disease).843 Although macroembolization and microembolization are major sources of stroke, hypoperfu- sion,844 perhaps in conjunction with embolization,845 is a risk factor for postoperative stroke. 