At 12:54 2004-10-26 -0700, Sandie Peters wrote:
[...] Several questions: 1. Why do we get a different ANOVA answer when we switch the order of the explanatory variables , e.g. ComponentA ~ Parent + Plot vs ComponentA ~ Plot + Parent? 2. Why are the Sum Sq and F ratios calculated by this ANOVA method different than those that are calculated by SAS JMP?
It's in the FAQ: http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-does-the-output-from-anova_0028_0029-depend-on-the-order-of-factors-in-the-model_003f
Also, see the `anova.lm' help page for a nice example.
If you want test based on so called Type II and III sums of squares, but don't want to compute them yourself (by use of `anova' and `drop1' as the FAQ suggests), John Fox's excellent `car' package has got an `Anova' function.
HTH, Henric
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