How can we make valid and useful comparisons of different health care systems?

It is important to understand and seek to reduce unwarranted variations in health treatments in order to improve health outcomes, inequalities in access and health system efficiency. Traditionally this monitoring function has been undertaken at national or subnational levels, as a means of identifying potential improvements in clinical practice and the performance of the health systems. However, international comparison of treatments is also recognized as being an important tool for assessing performance and prompting improvement, especially when examining whether the design of the health system needs reconsideration.1 However, making international comparisons is not straightforward, with two challenges standing out: first, the difficulty of making valid like-for-like comparisons; second, whether the analysis can help drive performance improvements.