You're right that you'd have to ask everybody, to get a completely accurate answer (although some of them will have changed their minds a week later).The numbers you get from polls will be within a few percent of the actual numbers 19 times out of 20. The more people you ask, the smaller the uncertainly becomes, but you'll be surprised how few people you have to ask in a population of millions to get within 2 or 3% of the correct answer 19 times out of 20.For the whole of the USA, for 2% margin of error, you only have to ask less than 2,500 people. (just over 4,000, if you want to be 99% sure it's within 2% of the result of the poll!) There's a tool, here:www.surveymonkey.com/mp/sample-size-calculator/
I'm sure most vegans are white people (in the US), but a larger proportion of mixed-race people are vegan. Probably, I guess, because they have less conservative parents.
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