Here's an excerpt from "Ill-Conceived Ranking Makes for Unhealthy Debate":
The WHO ranking was ambitious in its scope, grading each nation's health care on five factors. Two of these were relatively uncontroversial: health level, which is roughly the average healthy lifespan of a nation's residents; and responsiveness, which is a sort of customer-service rating encompassing factors such as the system's speed, choice and quality of amenities. The other three measure inequality in health-care outcomes; responsiveness; and individual spending.John Stossel makes similar criticisms of the WHO methodology here.
These last three measures struck some analysts as problematic, because a country with unhealthy people could rank above a healthier one where there was a bigger gap between healthy and unhealthy people. It is certainly possible that spreading health care as evenly as possible makes a society healthier, but the rankings struck some health-care researchers as assuming that, rather than demonstrating it.
An even bigger problem was shared by all five of these factors: The underlying data about each nation generally weren't available. So WHO researchers calculated the relationship between those factors and other, available numbers, such as literacy rates and income inequality. Such measures, they argued, were linked closely to health in those countries where fuller health data were available. Even though there was no way to be sure that link held in other countries, they used these literacy and income data to estimate health performance.