Glen: see below. On Fri, 28 Jun 2002, Glen Barnett wrote, in response to my comment:
> > Look in your favorite ANOVA text for the chapter on post hoc > > comparisons, and in particular the method of Scheffe' (called in some > > texts the "S-method"). The linear combination you describe is > > properly called a "contrast" in this context, a contrast being merely > > a linear combination of means with the constraint that the > > coefficients of the means sum to 0 (as your +0.5, +0.5, -1.0 do). > > A contrast needn't be a post hoc comparison. Often a particular contrast > is the primary comparison of interest right from the start. In that > circumstance, the Scheffe thing won't be appropriate. > > Glen Please re-read my complete response. The "t" value obtained is the same in any case; the only difference between an a priori test (or "planned comparison") and a post hoc test (Scheffe') is the critical value against which one compares the test statistic "t": in particular, whether one uses a critical value from "Student"'s t distribution or from (the square root of) Snedecor's F distribution with m-1 d.f. for the numerator. -- Don. ----------------------------------------------------------------------- Donald F. Burrill [EMAIL PROTECTED] 184 Nashua Road, Bedford, NH 03110 (603) 471-7128 . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
