Because self-selected populations make biased studies. If you're only looking at the people who a) join a FB group, b) see the post, c) respond to the post, and d) respond to the follow-up you're almost guaranteed to ignore anyone the intervention hasn't worked for
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It's like if you asked car enthusiasts whether driving was a good idea. The answer you get might be interesting, but hard to generalize to anyone else

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So can we ever ask how to fix a car by asking a car enthusiast?
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