[R] Dominant factors in aov?

Jonathan Baron baron at psych.upenn.edu
Thu Dec 2 16:17:41 CET 2004

I'm not a statistician, so take what I say with a grain of salt.

On 12/02/04 06:29, Rene Eschen wrote:
>Can someone explain me why factor C has only 25 Df (in stead of 28, what I
>expected), and why this number changes when I leave out factors B or C (but
>not A)? Why do factors B and C (but again: not A) not show up in the
>calculation if they appear later in the formula than D?
>When I ask summary.lm(aov(Y~A+B+C+D+E)), R tells me that three levels of D
>were not defined because of "singularities" (what does this word mean?).
>After checking and playing around with the dataset, I find no logical reason
>for which levels are not defined. Even if I construct a "perfect" dataset
>(balanced, no missing values) I never get the correct number of Df.

I would guess that the factors are somewhat predictable from each
other.  That is, there is some redundancy.  Try predicting each
factor from all the others, without the dependent variable.

Jonathan Baron, Professor of Psychology, University of Pennsylvania
Home page: http://www.sas.upenn.edu/~baron
R search page: http://finzi.psych.upenn.edu/

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