re: Re: st: post test comparisons of means using repeated measures.

[David Airey <david.airey@vanderbilt.edu>]

Joseph Coveney said,

> I've tried to compare both approaches in the do-file below using the 
> dataset
> that Alan cited.  The -mtest(bonferroni)- approach yields far more null
> hypothesis rejections than the corresponding Bonferroni-adjusted 
> separate
> t-test approach.  Such a discrepancy is surprising.
This happens in a oneway between-subjects design too. The reason the 
contrast is more powerful than the independent ttest is that the 
contrast uses information for MSw from all groups and is therefore more 
accurate. Of course this is true only if the homogeneity of variances 
assumption holds. Perhaps along the same vein, the contrasts reject the 
null more than the dependent ttests because more information is used 
for the estimate of error, and in this case homogeneity of differences 
variances holds? The dependent ttest is sometimes preferred because you 
can use information from only those means tested, which is useful when 
sphericity is violated.

-Dave

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