I think you are testing very different hypotheses in these two cases. If nothing else you've partitioned the SSbetween from the one-way ANOVA into three pieces indicating that you are testing three questions in the two-way ANOVA instead of one with the one-way.
Your effect for "A" will clearly be different because you are testing different means with the two-way than the one-way. Once you impose two levels of "B" you reduce the number of levels of "A" from 10 to 5. So what you are testing changes, fairly dramatically, so the p-values and F-values should change as well. Michael **************************************************** Michael Granaas [EMAIL PROTECTED] Assoc. Prof. Phone: 605 677 5295 Dept. of Psychology FAX: 605 677 3195 University of South Dakota 414 E. Clark St. Vermillion, SD 57069 ***************************************************** ----- Original Message ----- From: [EMAIL PROTECTED] (John Katsaridas) Date: Tuesday, March 30, 2004 7:18 am Subject: [edstat] one way ANOVA vs two way ANOVA > 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. > . > . > ================================================================= > Instructions for joining and leaving this list, remarks about the > problem of INAPPROPRIATE MESSAGES, and archives are available at: > . http://jse.stat.ncsu.edu/ . > ================================================================= > . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
