>
> Sigma is hardly ever known, so you must use t. Then why not simply tell
> the students: "use the t table as far as it goes, (usually around
> n=120), and after that, use the n=\infty line (which corresponds to the
> normal distribution). Then there is no need for a rule for "when to use
> z, when to use t".
>
but the data is not normal either in 99.9(9) of the cases. Furthermore,
the data that you see in economics/business is very often is not an iid
sample either. So, one way or another you end up with normal or
chi-square.
actually, there is an alternative to both approaches. it's bootstrap. but
it does not always work and should not be used blindly.
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