BRITAIN'S National Health Service (NHS) was recently judged the “world’s best health-care system” by the Washington-based Commonwealth Fund in its latest ranking of 11 rich countries’ health provision. The Commonwealth Fund tends to give the NHS a pretty clean bill of health in its assessments (it also scores Switzerland, Sweden and Australia highly). Other rankings reach different conclusions. How do you compare something as complex as a national health-care system with its peers? The Commonwealth Fund makes quality, access, value for money and equity the leading criteria for judging which countries perform well. Its emphasis on access and per-capita spending mean that America, struggling to extend its insurance coverage, while committing a large amount to overall health-care spending, regularly comes bottom of the Commonwealth Fund table. But that judgment overlooks what American health care delivers well: it scores highly on preventative health measures, patient-centred care and innovation, for instance. It has led the way in reducing avoidable harm to patients, with Seattle’s Virginia Mason hospital delivering “near zero harm”, something many systems, including England’s, are seeking to emulate.

What the NHS is good at is providing cost-efficient care. It spends $3,405 per person per annum, less than half America's outlay of $8,508. Alas, that does not mean the NHS is financially secure: a £2 billion ($3.4 billion) shortfall looms from 2015 and NHS England is struggling to implement £20 billion in savings. And some outcomes for serious conditions do not commend the English model, which does worse on serious cancer treatment than Canada, Australia and Sweden, according to data from the King’s Fund, a health-care think-tank based in London. American women have higher survival rates for breast cancer. Mortality rates following strokes also let down the English system.

Not everyone agrees with the Commonwealth Fund about what should be measured, and how the results should be weighted. A survey on health-care efficiency by Bloomberg recently chose Hong Kong, Singapore and Japan as the best performers, based on their efficiency. Adding greater weight to patient choice, for example, might reshuffle the rankings. The Commonwealth Fund most values equity and access, and so rewards the systems where it finds these. But change the weighting given to each category and you can quickly change the outcome. When it comes to judging the world’s health systems, preferences and values guide conclusions, as well as raw data.

Dig deeper:

Britain's health-care system, under fire at home, is doing better as an export (August 2013)

To improve health care, governments need better data (February 2014)

In a blind test, many Republicans actually support Obamacare (October 2013)