(Level of Evidence: C) See Online Data Supplement 32 for additional data on outcomes or volume as CABG quality measures. Numerous studies have demonstrated an association be- tween hospital or individual practitioner volume and outcome for a variety of surgical procedures and some medical conditions.805 831 CABG was one of the original procedures for which this volume outcome association was investi- gated.309,751,791 798,800 804,807,818 The CABG volume outcome association is generally weaker than that of other procedures, such as esophagectomy or pancreatectomy, which are per- formed less often. In addition, the results of volume outcome studies vary substantially according to methodology. The apparent strength of the volume outcome association often diminishes with proper risk adjustment based on clinical (as opposed to administrative) data.802,808 It is also weaker in more contemporary studies, presumably because of improved techniques and increasing experience.795,802 Finally, volume outcome associations appear weaker when hierarchical mod- els are used that properly account for small sample sizes and clustering of observations.832 The impact of CABG volume was studied in an observational cohort of 144 526 patients from 733 hospitals that participated in the STS Adult Cardiac Surgery Database in 2007.309 In this analysis, a weak associ- ation between volume and unadjusted mortality rate was noted (2.6% unadjusted mortality rate for hospitals perform- ing 100 procedures versus 1.7% for hospitals performing 450 procedures).309 Using multivariate hierarchical regres- sion, the largest OR (1.49) was found for the lowest-volume ( 100 cases) group versus the highest-volume group. Desir- able processes of care (except for use of the IMA) and morbidity rates were not associated with volume. 