Case mix adjustment in comparative audit

There is increasing pressure to measure and publicize the quality of health care, with, for example, hospital 'league tables' appearing regularly in the lay press. Equally, there is increasing recognition that such simplistic presentations of data can be seriously misleading, as differences in case mix mean that they do not compare like with like. We discuss what is required of a statistical model if it si to be used to make comparisons of clinical performance which are adjusted for case mix. Outcome measures must be well defined and relevant, and the risk factors used in the model must reflect the patients' potential prognosis rather than aspects of his or her clinical care. In addition, the calibration of the statistical model is crucial. We review a number of established clinical scoring systems and demonstrate that they fail to meet all of the required criteria. Of particular concern is the fact that these scoring systems are susceptible to biases which could actually mask differences in performance. We highlight areas where further research is required, with the aim of stimulating progress in this field.