Joe Galenko wrote:
> The mean of a random sample of size 81 from a population of size 1 billion
> is going to be Normally distributed regardless of the distribution of the
> overall population (i.e., the 1 billion). Oftentimes the magic number of
> 30 is used to say that the mean will have a Normal distribution, although
> that is when we're drawing from an infinitely large population. But for
> the purposes of determining the distribution of a mean, 1 billion is
> effectively infinite. And so, 81 is plenty.
There are other issues. First off, if the underlying population is not normal,
the sample mean will not be normal. Period. However, if the sample size
is large, the distribution of the sample mean becomes _approximately_
normal as the sample size increases. The question is at what point the
difference from a normal distribution becomes so small as to be negligible.
If the distribution is "nice", samples of size 10 may have reasonably
well-behaved sample means. On the other hand, if the population is
sufficiently awful, 200 points may not be enough. It just depends.
-------------------------------------------------------
gus gassmann ([EMAIL PROTECTED])
"When in doubt, travel."
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