In addition, standardized tools for symptom measurement and for patient subsets are generally lacking. For example, the response to the question, Did this procedure save your life? could be different for a patient undergoing PCI with an acute myocardial infarction and a patient with stable angina. In addition, validated risk-adjustment models for patient sur- vey data do not currently exist. Although the writing com- mittee believes that patient surveys are an important area for future development (see also Section 4.1.2: Patient Education/ Shared Decision Making Measures), these limitations raised concerns about their inclusion in the present document. 4.3. Structural Measures For PCI, measures to evaluate process and outcomes are more clearly substantiated by an evidence base than are structural measures. Still, compared with many clinically important process and outcome measures, it is easier to assess structural measures and, importantly, to track changes longitudinally without need for risk adjustment. Given these considerations, as well as interest in and evidence on registries and the role of case volume in outcomes, we elected to include 3 measures of structure: measure 9 (regional or national PCI registry par- ticipation), measure 10 (annual operator PCI volume) (quality improvement only), and measure 11 (annual hospital PCI vol- ume). It is the consensus of the writing committee that these structural measures can provide important contributions to the assessment of care equity and safety without imposing undue data collection burden on hospitals or practitioners. For both of the PCI case volume specific structural measures, existing standards encourage reporting.24 However, although the experi- ence of the operator and the hospital performing PCI has been associated with improved outcomes, it is not clear what spe- cific threshold volume of PCI cases represents a true clinically important indicator. 