@umbra21 - It's almost impossible to get a decent sample though. You'll always have to take the issue of it being a sample of convenience into account. Because even if you somehow manage to get a completely random sample it's still presumably going to be only from people in your country.
Which is one of the reasons scientists talk about "adjusting" the data.
That often gets misinterpreted when someone is trying to discredit the study, but it usually just means that they were trying to take these kinds of skewing factors into account.