How Can One Measure Happiness of a Nation?

How can Happiness be measured? Happiness is an emotion, and emotions can be very opinionated. And are people even reliable guides of their own happiness? What if they are being affected by severe depression and even though they have everything a chemical imbalance in their brain causes them to show severe emotions they wouldn't normally show such as extreme sadness in normal situations or in the case of Wilson's disease extreme happiness? How can one measure such an abstract idea with so many variables and put them together like a puzzle allowing them to compare different countries' happiness? Some people may suggest progress can correlate with happiness as progress leads to better lives for humans and in turn better moods but how can one measure progress than?