Measuring the quality of hospital care is a thorny business. Health care is complex, and links between clinical practice and patient outcomes are often tenuous and distant. These challenges have not prevented the pursuit of simple indicators that identify the “good” and “bad” hospitals. With the claim of providing such an indicator, the hospital standardised mortality ratio was launched with considerable fanfare—first in the United Kingdom and then in Europe and North America. The ratio identifies hospitals where more patients die than would be expected on the basis of their case mix—the bad hospitals—and hospitals with fewer deaths than expected—the good hospitals.1 2

The attraction of using the hospital standardised mortality ratio is clear. These ratios focus on a clear and important clinical outcome and use routinely available data which are as good at predicting death in some conditions as expensive clinical databases.3 However, their use as indicators of quality and …