Re: [R] Anova problem with order of terms in model

From: Berton Gunter <gunter.berton_at_gene.com>
Date: Wed 18 Jan 2006 - 08:13:34 EST


I suggest you consult a local statistician or read up on linear models,perhaps by reading the relevant section of V&R's MASS. Also search the R-Help archives for "Type III SS." Further comments below.

"The business of the statistician is to catalyze the scientific learning process." - George E. P. Box    

> -----Original Message-----
> From: r-help-bounces@stat.math.ethz.ch
> [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Martin Julien
> Sent: Tuesday, January 17, 2006 12:34 PM
> To: r-help@stat.math.ethz.ch
> Subject: [R] Anova problem with order of terms in model
>
> Hi
> I have a linear model and I want to tests whether the model terms are
> significant
> I used anova but F and P value depend on the order of the terms in the
> model.

Correct. Differennt hypotheses are being tested. R does **sequential** SS (this preserves additivity/orthogonality of contrasts being tested). In a non-orthogonal designs, these depend on order.

> I have repeated the same analysis in another stat software
> and F and P value
> did not differ with order of terms in the model.
> Can anyone can explain what happen or what am I doing wrong ?

And this "other software" is not doing a sequential SS, but rather what is usually called Type III SS (google on "Type III SS" for more info).



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