Why health care process performance measures can have different relationships to outcomes for patients and hospitals: understanding the ecological fallacy.

Relationships between health care process performance measures (PPMs) and outcomes can differ in magnitude and even direction for patients versus higher level units (e.g., health care facilities). Such discrepancies can arise because facility-level relationships ignore PPM-outcome relationships for patients within facilities, may have different confounders than patient-level PPM-outcome relationships, and may reflect facility effect modification of patient PPM-outcome relationships. If a patient-level PPM is related to better patient outcomes, that care process should be encouraged. However, the finding in a multilevel analysis that the proportion of patients receiving PPM care across facilities nevertheless is linked to poor hospital outcomes would suggest that interventions targeting the health care facility also are needed.