Oneway ANOVA w/ 3 groups
I just had a conceptual question.
With 3 or more groups, what does the significance in the "Tests of Between-Subjects Effects" table in the IV row represent? If significant, does it mean that at least one of the pairwise comparisons have significantly different means?
Or is it possible that even if the F of the IV is not significant, the post hoc tests may show one or more of the differences to be significant? Or vice versa (significant F results in non-significant post hoc differences)?
I guess I'm asking about the relationship between the significance in the "Tests of Between-Subjects Effects" table versus the post hoc "Multiple Comparisons" table.
Thanks!
posted by David @ 1:26 AM
1 Comments:
At 5:18 PM,
Mari said…
The omnibus F (i.e., the F in the Test of Between Subjects Effects) tells you that, when you look at all groups together, there is a systematic group effect for the IV you are testing.
The post-hocs in the Multiple Comparison Procedures tests tell you whether or not there are significant pairwise differences between groups. Thus, the post-hocs do NOT consider all groups together, but one pair of groups at a time.
It is, in fact, possible to have a significant overall (or omnibus) F but not have significant results on your post-hocs. (This is more common when testing interactions, but can occur in oneway analyses.)
You should NEVER, NEVER, NEVER run post-hocs in the absence of a significant omnibus F, but it is possible to have pairwise comparisons that reach significance even when the overall effect is not significantly different from chance. Note that examining these post-hocs in the absence of a significant omnibus F is akin to running multiple t tests. The actual alpha level in this case is not < .05.
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