Hi On Thu, 15 Nov 2001, Jerry Dallal wrote: > But, if the null hypothesis is that the means are the same, why > isn't(aren't) the sample variance(s) calculated about a pooled > estimate of the common mean?
What you are testing is whether there is more variability between groups than you would expect by chance given the variability within groups. This is most clear with the F test, of course, (i.e., F = n*Vmeans/Vwithin) but t is simply a variation of this. Is the difference between X1 and X2 (i.e., variation in Xjs) greater than expected given variation within groups. Taking the common mean to calculate a variance would conflate the within and between group factors that you want to contrast. Best wishes Jim ============================================================================ James M. Clark (204) 786-9757 Department of Psychology (204) 774-4134 Fax University of Winnipeg 4L05D Winnipeg, Manitoba R3B 2E9 [EMAIL PROTECTED] CANADA http://www.uwinnipeg.ca/~clark ============================================================================ ================================================================= Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =================================================================
