"Anybody can take the survey. Hopefully this won't skew the results."

No. The introduction tells you who it is he is surveying.

And, of course, people falsely claiming to qualify would skew the results. If I participated in a questionnaire about the longterm effects of being a smoker, my status as a non-smoker WOULD skew the results. Why, if I cared about the study, would I want to undermine it?

Dante