Sample size in a voluntary survey: the selection problem

Posting this under the heading it deserves: Sample size in a voluntary survey: the selection problem Everything below is what sits behind that.

Sample size: setting out the arithmetic in full, because I have had to do it twice and I would rather nobody else did.

Every step is shown. If the answer is wrong the error will be visible, which is the point of writing it out rather than posting the result.

That reframing is the whole thing. The facts I already had.

Worth separating two things that post #3 runs together.

Aggregating first-hand accounts does not produce evidence of the kind a trial produces. It produces a description of who chose to post, which is a real thing and a different thing.

The answer changed when I changed how I was measuring, which was informative.

Units in the column header, always, and the same units down the whole column. Mixed units in one field is the commonest defect in shared spreadsheets here.

A single observation, in a thread that deserves better than single observations.

Marking my uncertainty on sample size explicitly. I am confident about the direction, much less confident about the size, and not confident at all that it generalises past the case in the first post.