The article has a paywall for me and I was curious about their methodology. Fortunately, Wikipedia has some information:
"Nationally representative samples of respondents are asked to think of a ladder, with the best possible life for them being a 10, and the worst possible life being a 0. They are then asked to rate their own current lives on that 0 to 10 scale."
"Nationally representative samples of respondents are asked to think of a ladder, with the best possible life for them being a 10, and the worst possible life being a 0. They are then asked to rate their own current lives on that 0 to 10 scale."
https://en.wikipedia.org/wiki/World_Happiness_Report