If we had a class of 200 people and we made a survey in which anyone in the class can choose to participate or not, does this count as random sampling?
I feel like it's not, but I can't put it into words why it is not.
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1 Answer
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It is not random sampling, because people have a free and possibly biased choice, or reason, to not participate. Put differently, the people opting-in are possibly different (in relevant characteristics) from those opting-out.
See also Wikipedia: self-selection bias.
