The idea of measuring happiness — or well-being — might seem a slippery undertaking. What are the metrics of such an intangible? Ramp up the concept to measuring the happiness of an entire country, and then to comparing the happiness of different countries, and you have an inscrutable challenge for economists — or so it might seem. It turns out there are some unlikely — even prominent — advocates for a happiness index who would like the residents of their countries to be the lucky ducks who live in an environment that encourages well-being along with economic success. They include U.K. Prime Minister David Cameron, who last year said the government will create a national well-being index. Cameron acknowledged that the concept of measuring happiness could easily be viewed as “airy-fairy and impractical,” yet nevertheless thinks it is important to do.…