On 19 Dec 2002 04:55:35 -0800, [EMAIL PROTECTED] (Ivan Balducci) wrote: > Hi members, > Please, may someone explain to me this doubt: > why is not right to perform a Tukey HSD test > after an ANOVA came out non-significant ? > Why Tukey HSD test should be done only following an ANOVA that was > significant ? > Which is the reason ?
If you are doing LSD (simple t tests) as followup testing, it is "followup": By definition, you need the overall test if you are going to protect against excessive alpha error. I don't know which of the other procedures are actually 'posterior tests' in this same rigorous sense. As Zar says, "Although not actually required by theory, multiple comparison testing is most commonly performed only if an analysis of variance first rejects a multisample hypothesis of equal means." [Biostatistical Analysis, 1999.] For most studies, I think it is going to be a pretty-much- irrelevant matter of caution, since the extra step won't make any difference. (In the instance of the Sheffe tests, it *can't* make a difference in the usual direction. Sheffe's insists that each pair can account for an overall effect. On the other hand, you might have an overall effect and *not* see any of your pairs look significant by Sheffe's.) For the Tukey HSD test, the reason is mainly precedent, combined with that caution, or with the statistical illiteracy. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
