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Date:         Mon, 5 Sep 2005 15:19:24 -0400
Reply-To:     "Frank J. Gallo" <fjgallo@verizon.net>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         "Frank J. Gallo" <fjgallo@verizon.net>
Subject:      ANOVA and Post-hoc Tests
Content-Type: text/plain; charset="US-ASCII"

Hi All,

Looking for some help on interpreting contradictions found in an ANOVA run with TukeyHSD follow-up. The F-test indicates significant differences among groups.

ANOVA

Sum of Squares

df

Mean Square

F

Sig.

Between Groups 2.849

3

.950

2.830

.041

Within Groups 46.649

139

.336

Total 49.498

142

Levene = 5.168, p=.002, the null hypothesis that the error variance of the dependent variable is equal across groups suggests no.

But, post-hoc comparisons show no significant differences among groups.

And, the homogenous subsets table identifies one set (4 groups, p=.143) indicating that the means of the subgroups are not significantly different from each other.

I do have unequal sample sizes: 23, 42, 30, 48. The run used the harmonic mean (32.934), so I am aware that the Type 1 error level is not guaranteed. Is this what I am seeing? That is, the p-value for the F-test is underestimated and it should be larger. This is my first experience with this kind of situation.

Yours thoughts are greatly appreciated.

Frank


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