On Mon, 19 Mar 2001 13:14:39 -0500, Bruce Weaver
<[EMAIL PROTECTED]> wrote:
> On Fri, 16 Mar 2001, Rich Ulrich wrote:
[ snip, including earlier post ]
> > That ANOVA is inherently a 2-sided test. So is the traditional 2x2
> > contingency table. That is because, sides refer to hypotheses.
>
> <snip>
>
>
> I agree with you Rich, except that I don't find "2-sided" all that
> appropriate for describing ANOVA. For an ANOVA with more than 2 groups,
> there are MULTIPLE patterns of means that invalidate the null hypothesis,
> not just 2. With only 3 groups,for example:
>
> A < B < C
> A < C < B
> B < A < C
[ ... ]
> And then if you included all of the cases where 2 of the means are equal
> to each other, but not equal to the 3rd mean, there are several more
> possibilities. And these ways of departing from 3 equal means do not
> correspond to tails in some distribution.
>
> There's my attempt to add to the confusion. ;-)
If I convince people that they want only one *contrast* for their
ANOVA, then it is just two-sided. I've been talking people out
of blindly testing multiple-groups and multiple periods, for years.
Then I have to start over on the folks, to convince them about
MANOVA. If there are two groups and two variables,
there are FOUR sides -- and that's if you just count what is
'significant' by the single variables. Most of the possible results
are not useful ones; that is, they are not easily interpretable, when
no variable is 'significant' by itself, or when logical directions
seem to conflict.
We can interpret "group A is better than B." And we analyze
measures that have the scaled meaning, where one end is better.
So the sensible analysis uses a defined contrast, the 'composite
score'; and then you don't have to use the MANOVA packages,
and you have the improved power of testing just one or two sides.
--
Rich Ulrich, [EMAIL PROTECTED]
http://www.pitt.edu/~wpilib/index.html
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