rghuang wrote (excerpted):
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. . . should we use ttest when we don't know if the variances is equal?
so whether should we use sdtest to find if the variances is equal?if not
equal,should should i do if i want to see whether the mean for write is the
same for males and females?
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-ttest- has two options for when the variances are not assumed to be equal:
Satterthwaite adjustment of the degrees of freedom (-ttest , unequal-) and
Welch adjustment of the degrees of freedom (-ttest , unequal welch-).
I believe that it is not widely recommended to test for equality of variances
before applying Student's t test. If the ratio of variances is disturbing,
then you can either apply the Satterthwaite or Welch approximation, or consider
using a permutation t-test (-findit permt2-).
Joseph Coveney
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