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National Hospital Quality Measures (NHQM) should accurately reflect quality of care, as they increasingly impact reimbursement and reputation. However, similar to risk adjustment of outcomes measures, NHQM process measures pose unique methodological concerns, including lack of representativeness of the final denominator population after exclusions. This study determines population size and characteristics for each acute myocardial infarction (AMI) measure, reasons for exclusion from the measures, and variation in exclusion rates among hospitals.163 144 discharges from 172 University HealthSystem Consortium hospitals between 2008-Q4 and 2013-Q3 were examined, including characteristics and propensity scores of included and excluded groups. Measure exclusions ranged from 17.8% (discharge aspirin) to 90.1% (percutaneous coronary intervention, PCI, within 90 min), with substantial variation across hospitals. Median annual denominator size (IQR) for PCI within 90 min was 28 (20, 44) at major teaching hospitals, versus 10 (0, 25) at non-teaching hospitals. Patients most likely to be excluded (in the 10th vs 1st propensity decile) were older (mean age (SD) of 78.1 (10.8) vs 50.3 (8.6) years), more likely to have Medicare (90.5% vs 0.9%), had more documented comorbidities (15.6 (4.6) vs 6.2 (2.5) hierarchical clinical condition categories) and higher admission mortality risk (Major or Extreme 80.9% vs 7.3%, respectively), and experienced higher inpatient mortality (10.0% vs 1.6%).Exclusion from AMI measures varied substantially among hospitals, sample sizes were very small for some measures (PCI and ACE inhibitor measures) and measures often excluded high-risk populations. This has implications for the representativeness and comparability of the measures and provides insight for future measure development.