so it's all done by serveys?
When you say it that way, I imagine some guy standing out at the supermaket asking each person who walks by what their nationality is. Most likely, when people register to vote, buy property/a house, apply for citizenship, are issued a birth certificate, and/or get married, they are asked what nationality they are a descendant of. Of course there are always a few mistakes, but when you get 1 billion right and 2,000 wrong, it's not that big of a deal. We aren't calculating something to save the world from doom, we're just curious as to where all the Japanese emmigrated to. B)
btw, if you are not japanese by blood but by culture and language, does that still make you japanese?
If you were born with Japanese citizenship or a permanent resident visa, you'd be a second-generation American Japanese.