Case mix

Case mix describes the distribution of patient characteristics affecting prognosis, such as age, severity, comorbidity and urgency. Comparing provider outcomes without accounting for case mix may confuse differences in patients with differences in care. Adjustment requires comparable, reliably measured baseline variables and does not remove all possible confounding.

A risk-adjusted provider comparison contrasts observed outcomes with outcomes expected from a prespecified patient-risk model, or standardizes providers to a common patient population. For individual fitted risks $p_i$, the expected event count is ${\sum }_i{p}_i$. Patient selection, model calibration, omitted risk factors, small samples and dependence between outcomes can still affect interpretation. Funnel plots display precision differences that an ordered league table can obscure.