Posted by
Mike on
Jan 02, 2014; 4:59pm
URL: http://spssx-discussion.165.s1.nabble.com/One-Way-Anova-Repeated-Measures-Main-effect-is-significant-but-post-hoc-tests-are-not-significant-tp5723746p5723747.html
You may have gotten nonsignificant Bonferroni results because:
(1) you lack statistical power to detect differences. Consider
specifying LSD post hoc comparisons to see if any of the
differences are significant. The problem with this approach is
that you no longer have overall alpha = .05. But this could
indicate where the differences are if you increase the number
of subjects.
(2) the post hoc tests compare the means on a pairwise basis
(i.e., one mean vs another mean) but the significant ANOVA
only indicates that *some* combination of means are different.
You don't specify how many levels you have but if you have
four means, it is possible that [(mean 1 + mean2)/2] - [(mean3 -
mean4)/2] is the combination that is statistically significant.
Kirk's textbook of Experimental Design has a good chapter
on planned comparisons/post hoc tests that you might consider
looking at.
-Mike Palij
New York University
[hidden email]
P.S. The Dunn-Sidak version of the Bonferroni test is supposed
to be somewhat more powerful than ordinary Bonferroni test, so
you might also consider this if you focus is only on pairwise
comparisons.
----- Original Message -----
From: "JohnDoe" <
[hidden email]>
To: <
[hidden email]>
Sent: Thursday, January 02, 2014 11:35 AM
Subject: One-Way Anova Repeated Measures: Main effect is significant,
but post-hoc tests are not significant
> Hi,
>
> I ran a one-way within-subjects ANOVA, with equal group sizes, and my
> data
> met the assumption of sphericity. I found a significant Main effect,
> but
> Bonferroni's tests did not reveal any significant post-hoc effects.
>
> Does anyone know why this might have happened? And how would I report
> this
> result?
>
> Thanks very much
>
>
>
> --
> View this message in context:
>
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