# ANOVA and Scheffe's post hoc test

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## ANOVA and Scheffe's post hoc test

 Dear Lister, I am using one way ANOVA followed by Scheffe's post hoc test for testing some hypotheses. Sometimes ANOVA gives significant results but all paired comparison results of Scheffe's test are not significnat. Why does this happen? Suppost out of 10 paired comparisons, I get significant results at 4 combinations, but not significnat at rest six combinations. Should I say that hypothesis is largely rejected? I have read that Scheffe's post hoc test is the most conservative test among all post hoc test for ANOVA. To what extent I am right? -- Regards Pushpender Nath Saini
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## Re: ANOVA and Scheffe's post hoc test

 Hi Pushpender Nath Saini Friday, June 1, 2007, 7:16:39 AM, You wrote: PN> Dear Lister, PN> I am using one way ANOVA followed by Scheffe's post hoc test for testing PN> some hypotheses. Sometimes ANOVA gives significant results but all paired PN> comparison results of Scheffe's test are not significnat. Why does this PN> happen? First, when you say "paired", do you mean "pairwise"? (quite different idea). Assuming you meant "pairwise", here's the reply: Because, answering the question you ask below, Schefee's post hoc is the MOST conservative test. Use Tukey's HSD instead (good control of the experimentwise type I error, but still a bit sensitive) PN> Suppost out of 10 paired comparisons, pairwise? PN> I get significant results at 4 PN> combinations, but not significnat at rest six combinations. Should I say PN> that hypothesis is largely rejected? NO. The ANOVA overall test is used to reject the null hypothesis (equality of ALL means). Once you have rejected it (simply "rejected", not "largely" or "quite" or "scarcely"...), use a post hoc test to find out which groups are significantly different from others. PN> I have read that Scheffe's post hoc test is the most conservative test among PN> all post hoc test for ANOVA. To what extent I am right? The MOST conservative, as a matter of fact. Why did you choose it? Are you very worried about the chance of a type I error? This test will protect you against it, but, as you have seen with your data, you will probably be faced with type II errors, due to the lack of power of this method. -- Regards, Dr. Marta García-Granero,PhD           mailto:[hidden email] Statistician --- "It is unwise to use a statistical procedure whose use one does not understand. SPSS syntax guide cannot supply this knowledge, and it is certainly no substitute for the basic understanding of statistics and statistical thinking that is essential for the wise choice of methods and the correct interpretation of their results". (Adapted from WinPepi manual - I'm sure Joe Abrahmson will not mind)
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