Each matched pair contained one patient from a high-volume hospital and one from a low-volume hospital (N=3,151 MM pairs and N=6,387 non-MM pairs), matched exactly on 135 principal procedures and whether or not the patient was multimorbid, and further matched on risk of mortality on admission, a propensity score for admission to a low-volume hospital, demographics, admission type, 29 comorbidities, and 113 individual qualifying comorbidity sets.