Risk-adjusted measurement of primary cesarean sections: reliable assessment of the quality of obstetrical services.

A two-hospital system reported widely disparate Cesarean section rates in its component institutions. Statistical analysis determined that the apparent discrepancy was due primarily to patient-related factors. When risk-adjusted, both hospitals' rates were indistinguishable from expected rates. Reporting Cesarean section rates without appropriate risk adjustment yields potentially misleading results. Since reliable risk adjustment currently exists only for primary Cesarean sections, primary rates should be reported separately from "raw" rates for other procedures.