Simpson's paradox: how performance measurement can fail even with perfect risk adjustment

Efforts to measure quality using patient outcomes—whether hospital mortality rates or major complication rates for individual surgery—often become mired in debates over the adequacy of adjustment for case-mix. Some hospitals take care of sicker patients than other hospitals. Some surgeons operate on patients whom other surgeons feel exceed their skill levels. We do not want to penalise hospitals or doctors who accept referrals for more complex...