"Robert J. MacG. Dawson" wrote:
> Oh, it never is (strictly), outside of a few industrial
> applications. Nobody ever took a random equal-probability sample from
> all turnips, all cancer patients, all batches of stainless steel, all
> white mice, or all squirrels. However, there are good common-sense
> reasons to believe that *some* convenience samples will act enough like
> true random samples to be useful. Otherwise we could stop teaching
> statistics to biology and psychology students.
I agree with pretty much everything you said, however, I think the
defence of convenience samples can be stronger than this. Unless we
have reason to believe that a sample is biased in such a way as to
generate our pattern of results a convenience sample is just as good
evidence as a (hypothetical) random sample. In some cases (e.g., a
convenience sample we think, a priori, should be biased against the
observed pattern) it might constitute stronger evidence than a
random sample. For example, if a new drug is administered to a
treatment group made up of serious cases and compared to a control
group of mild cases obtaining more "cures" for the treatment group
might be considered better evidence than a random sample.
Thom
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