7, 2014 Nallamothu et al. February 25, 2014:722 45 ACC/AHA/SCAI/AMA-PCPI/NCQA 4.2.1. Level of Attribution/Aggregation Contributions of multiple healthcare providers across mul- tiple settings are re ected in outcomes associated with any particular episode of care, and this can be especially true in the case of PCI. In addition, various data sources and data systems are the window into that episode, such that the ability to aggregate data at the level of an individual clinician versus a broader grouping (e.g., practice or hospital) will depend on the types of data available and the outcomes being evaluated. Although data are increasingly available, most sources of information, like administrative claims data, generally lack adequate granularity to be of meaningful use for attribution of outcomes performance at the level of the individual provider, which makes aggregation of PCI out- comes more appropriate for the health system or hospital. 4.2.2. Infrequently Occurring Complications Certain outcomes could be of inarguable importance in PCI but occur rarely. Such outcomes are dif cult to interpret at the individual-provider level simply because of the fact that low-frequency events in a small sample size will produce unreliable estimates of provider performance. For this reason, certain measures are appropriately applied only to larger aggregated provider groupings where sample sizes are larger. These principles have substantial implications for PCI outcomes because the rates of major complications, such as death and the need for emergency coronary artery bypass surgery, have decreased signi cantly in recent years. Death is perhaps the most important and least ambiguous outcome measure. Proper risk adjustment isdand will remainda mandatory cornerstone of mortality monitoring for PCI. However, the writing committee also recognized that even the best risk-adjustment model cannot correct for potentially unmeasured confounders, and most risk- adjustment models perform less well at the extremes of risk. 