GS, there are many agencies, including everyting from the UN to census bureaus to advertising companies, that keep track of population figures and demographics. Get three or four good world almanacs together and compare figures. They may vary a smidgen here and there, but if you take your figures from any one of them, or average them, and boil them down to percentages (which, essentially, is what that web site does), those are the figures you'll come up with.