On 24 October 2010 07:32, Helen Mann <[log in to unmask]> wrote:
> ooops made a mistake in the last email....
>
>
> Me again....Two things this time....
>
> 1) Can someone tell me why when I did a Wilcoxon Signed Ranks test
> (non-para) I found signficant differences between a) time 1-time 2 b) time
> 1-time 3 c) time 2 and time 3
> BUT when I did the parametric alternative (repeated measures ANOVA) I got
> significant differences on a) time 1-time 2 b) time 1-time 3 but c) time 2
> and time 3 was not significant....
>
> I thought that as non-para tests are not as sensitive as para tests that it
> would've been the other way round, that the para test would show something
> significant that the non-para wouldn't find.....
>
> Any ideas?
>
Hard to say without playing with the data. Parametric tests are more
powerful if all the assumptions are satisfied. But if the assumptions
are not satisfied, they can be less powerful, or they can be wrong.
> 2) Given a data set of 155 participants on before, pre- and post- test (time
> 1, time 2 and time 3) what reason(s) would you choose to do a Friedman and
> Wilcoxon Signed Ranks instead of the repeated measures ANOVA when all your
> other tests have been parametric?
>
> Last minute panic before the viva on Friday!!
>
Depends what other tests you did. RM Anova makes additional
assumptions about things like sphericity that other anovas don't make.
Personally, I don't like Friedman's tests and would do (almost)
anything to do a different test.
J
--
Jeremy Miles
Psychology Research Methods Wiki: www.researchmethodsinpsychology.com
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