Hi On 30 Mar 2004, John Katsaridas wrote: > I am using Excel to do some ANOVA calculations. Say that we have one > independent variable A, where we have 10 samples for each level of A. > Then a one way ANOVA tells us whether there is a significant > difference > in the means of A's levels. > If I add a second independent variable B with 2 levels to the data > set, > and I assign 5 samples from each level of A to each level of B, then > each cell of the 2 dimensional table will be made up of 5 samples. > When I perform a two > way ANOVA in Excel, it gives us 3 values, then means of the samples > grouped > by factor A, the means grouped by factor B, and the interaction > effect. > However the p value I get for the samples grouped by A in two way > ANOVA, > is different than the p value of the one way ANOVA on A that I > performed > at the beginning. Why is this? Shouldn't the p-values be identical? > Aren't > we testing the same hypothesis in each case? > Thank you very much.
Your first anova partitioned SStotal into SS(A) and SS(SwA), with the latter representing variability of Subjects within each of the levels of A, and it being the basis for the denominator of your F ratio. Your second anova, which introduced B, partitioned SStotal into SS(A), SS(B), SS(AxB), and SS(SwAB), with the last of these becoming the error term. Although SS(A) is the same in both analyses, SS(SwA) will generally be somewhat to much larger than SS(SwAB) and it will have more degrees of freedom. So the two F ratios would normally differ from each other, and could differ by quite a bit depending on the amount of variability associated with B and AxB (i.e., the variation subtracted from SS(SwA). 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, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
