Although the Demographics screen isn't really useful, many people are curious as to what those numbers really mean. Many people had theories and could figure out by themselves how some of them were figured.
Here is my analysis after extensive testing in the debug mode for PTW:
Approval rating: This one's easy. The percentage of your people that are happy. If every single person is happy, you have 100%. If everyone is content it is 50%. Edit: This can be misleading when you get specialists, because the specialists count as only content people. So even if everyone is happy or an entertainer, you won't have 100% approval rating if you have any specialists.
Population: Add up all the population you get from the city view from all your cities. Not population points, like size 1, 2 or 3, etc. but the 10,000 or 100,000 you see under the city name.
GNP: Total gold in all your cities before corruption takes a bite out of it.
1 gold= 1 million
