Free access. All healthcare systems show variation in the quality of care provided, whether that means access to primary care services,1 ambulance response times,2 Accident & Emergency waiting times3 or treatment processes and outcomes.4–6 Monitoring this variation in quality can serve multiple purposes: informing patients about where best to seek care;7 allowing clinicians to compare their performance with that of their peers and thus identify targets for local-level quality improvement efforts, and supporting the development of national policy. Though, what all these have in common is a trust in the reliability of the data to adequately reflect healthcare quality—sometimes a questionable assumption.

In BMJ Quality and Safety, Hofstede et al 8 have addressed a common situation where providers (such as hospitals, general practices or community teams) are ranked according to their performance on a quality indicator.