Re: one way anova overall significant, but dunnett post hoc has no single significant group?

Posted by Ware, William B on Oct 18, 2012; 11:30am
URL: http://spssx-discussion.165.s1.nabble.com/one-way-anova-overall-significant-but-dunnett-post-hoc-has-no-single-significant-group-tp5715709p5715725.html

The ANOVA is sometimes referred to as the "omnibus" test as it is testing the null hypothesis against a broad spectrum of alternate hypotheses.  I suspect that the initial ANOVA result is picking up something not addressed by the specific Dunnett contrasts, something like the average of the three groups being different from the control.  Or perhaps the average of two groups being different than another group...

wbw

William B. Ware, Ph.D.
McMichael Professor of Education 2011-2013
Educational Psychology, Measurement, and Evaluation
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University of North Carolina at Chapel Hill
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Adjunct Professor, School of Social Work
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-----Original Message-----
From: SPSSX(r) Discussion [mailto:[hidden email]] On Behalf Of chengfoh
Sent: Thursday, October 18, 2012 6:19 AM
To: [hidden email]
Subject: one way anova overall significant, but dunnett post hoc has no single significant group?

Hi hope someone can really help, this confuse me alot

I would like to compare the mean for 3 test groups to a control group fro different parameters (RBC, ALT, ALP....), so i ran a oneway ANOVA and then i conduct the post hoc using Dunnett's test..then i observed someting confusing me, whereby

There were parameter whereby when ANOVA overall P value was significant, however dunnett test gave no no single group with significant value, how possible?

Also, there were situations where overall ANOVA test was not significant, but posthoc (which i included sometime for test run) show some test groups with significant P value as compared to contorl...

Wish someone came across with these can help me, many thanks
Note: the number of subject in each group might differ, for example, each group can have 3/4 animals and is not throughouhly fixed



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