The seminal work of Wennberg and Gittelsohn in 1973 emphasised the importance of health information for informed decision-making. This led to the creation of the Dartmouth Health Atlas in 1996, which has become an important resource for monitoring health services in the USA. The Dartmouth Health Atlas research revealed the existence of variation in health care without benefit to patients, and the dependence of health care use on local resource supply. Similar initiatives emerged around the world, from the UK to Asia. The availability of administrative data has become essential for evaluating health service delivery and for informing health economic analysis and policy decisions. Access to data depends on the organisation of the health system, with more centralised systems facilitating comprehensive data collection. We contrast the decentralised structure of the Swiss healthcare system with that of the US and the UK, and highlight the challenges of harmonising data for nationwide health monitoring. The example of optical coherence tomography (OCT) in Swiss ophthalmology illustrates the variability in care practices and billing patterns. This variability can be attributed to the lack of clear guidelines and the complexity of billing codes. Incentives to charge incorrect rates influence billing, adding a further variance component to the variance in care that cannot be subtracted from the total variance at the level of a health insurance fund and distorting the results. In certain environments the quality of data on care is so variable that a sound conclusions for health policy decisions represent a great challenge.