Hi
On 2 Apr 2001, Will Hopkins wrote:
> in the within-subject error between levels of the factor. Fine, but
> then you want to do a specific contrast between levels of the
> within-subject factor, such as the last pre-treatment vs the first
> post-treatment (with or without a control group--it doesn't matter).
> Now, the p value you get for that contrast... is it based on the
> overall adjusted error derived from ALL levels of the
> repeated-measures factor, or is it nothing more than the p value for
> a t test of the two levels in question?
By default in SPSS, the error term used to test the significance
of each contrast is specific to the particular contrast. So with
a two-group comparison, it amounts to a paired-difference t-test
(or t^2 if reported as F). This characterization is general, and
is not specific to two-group comparisons. That is, for any
contrast it is possible to calculate individual contrast scores,
which will have a mean and contrast-specific variability. The
deviation of the mean contrast score from zero relative to its
standard error is what is tested for significance. The values so
calculated will agree with the Fs in the standard (SPSS) Anova
(SSs will also agree if normalized coefficients are used).
Whether other packages adopt the same approach or default to
other error terms, I do not know.
Best wishes
Jim
============================================================================
James M. Clark (204) 786-9757
Department of Psychology (204) 774-4134 Fax
University of Winnipeg 4L05D
Winnipeg, Manitoba R3B 2E9 [EMAIL PROTECTED]
CANADA http://www.uwinnipeg.ca/~clark
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