How do you measure national income? and why do countries need to measure it?

What is the single best measure for comparing different countries relative economic success?

• Are per capita measures better than aggregate measures? Should nominal or "purchasing power parity" measures be preferred?  Should GNP based measures be preferred to GDP based measures? Regarding individuals experience of success, is rate of improvement more important than absolute level of income/production? Is economic inequality within the country relavant?  Should you focus on the success of the "median" individual rather than "average" per capita measure to avoid being thrown off by uber-rich outliers? How should you account for differences in hours worked? I realize the answer to this question might "depend" on goals and values, so feel free to discuss what goals/values make you prefer one measure over another. This question is focused on economic success.  There are lots of things more important than economics (human rights, happiness, human dignity, etc.) however for the purpose of this question lets focus on answers relevant to economics.

• Answer:

  What are you trying to measure? Economic Success doesn't really mean anything other than that some number is increasing. Both GNP and GDP are thoroughly discredited as a measure of success, since they measure "bad" as well as "good" economic activity, and also change rapidly when informal activity is replaced with paid. (If you look after your neighbors kids and they cook for you then GDP/GNP goes up). I would also be wary of measuring anything in a developing country that uses averages or total (like GNP/GDP) rather than looking at the median, since a largely informal economy can see huge numeric changes if the 1%'s economic activity increases without any real change in economic activity as seen by the country as a whole.