Patient safety remains one of the most prominent issues in health policy and public debate. High rates of error during the delivery of medical care have been demonstrated repeatedly, including the landmark report by the Institute of Medicine which estimated that more people die from medical errors than from traffic injuries or breast cancer (Kohn et al., 2000). Robust comparison of performance with peers is fundamental to securing improvement. Two types of patient safety event can be distinguished for this purpose: never events, those events that should never occur, such as failure to remove surgical foreign bodies at the end of a procedure; and adverse events, such as post-operative sepsis, which can not be avoided in all cases given the high-risk nature of some procedures, although increased incidence at an aggregate level may indicate a systemic problem.