On 10/04/13 06:04, Alan Gauld wrote:
On 09/04/13 13:47, Steven D'Aprano wrote:
Since when did 30 become a representative sample size?
If they are randomly selected, 30 is likely plenty for a representative
sample size. In surveys, a sample size of 30 gives you a margin of error
of about 15%, which isn't too bad.
Hmm, if I'd gone to my manager (and especially the finance director) with a
project proposal based on assumptions with a 15% margin they'd have thrown me
out.
The 15% figure comes from surveys where people are asked to rate something on
some scale, say from 1 to 7. If your sample of 30 gives an average rating of 5,
then a 15% margin of error corresponds to plus or minus 1: the true average is
likely to be 4, 5 or 6. Given that the question being asking is fundamentally
subjective, and you have no idea whether Fred's rating of 5 and Wilma's rating
of 5 mean the same thing, I think that plus or minus 1 is pretty much as good
as you can reasonably expect. Additional samples just adds spurious precision
that looks good but doesn't *mean* anything.
But what do I know? :)
My suspicion is they just asked around their local department!
But maybe I'm just overly suspicious...
Nah, I reckon you've hit the nail right on the head.
--
Steven
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