Patient safety has been high on the national and international agenda in health care for almost a decade. In the United Kingdom, reviews of case records have shown that over 10% of patients experience an adverse event while in hospital,1 2 a figure reflected in similar studies around the world.3 Considerable efforts have been made to improve safety, and it is natural to ask whether these efforts have been well directed. Are patients any safer? The answer to this simple question is curiously elusive. Although some aspects of safety are difficult to measure for technical reasons (defining preventability for instance), the main problem is that measurement and evaluation have not been high on the agenda. We believe that the lack of reliable information on safety and quality of care is hindering improvement in safety across the world.

The principal approach to patient safety in the UK, United States, and many other countries has been to establish local and national reporting systems; these systems invite voluntary reporting of unspecified safety incidents with the aim of learning lessons and feeding back the findings into the system. However, these reporting systems do not effectively detect adverse events. In the most recent comparison, reporting systems detected only about 6% of adverse events found by systematic review of records.2 Reporting systems are a valuable component of a safety system, but they are essentially systems for warning and communication inside an organisation and, if large scale, of detecting rare events not easily detectable by other means. They cannot and never will act as a measurement system for safety.

Here, we use the example of the UK …