[R] type III sums of squares in R

[Mike Sears]

If you use the Anova function in the car package and specify contr.sum or 
contr.SAS for the contrasts for your categorical factors, you will get the 
same results as outputted by SAS. I've tried this with a variety of data sets 
and it works.

On Thu November 24 2005 08:27, Stefanie von Felten, IPW&IfU wrote:
> Hi everyone,
>
> Can someone explain me how to calculate SAS type III sums of squares in
> R? Not that I would like to use them, I know they are problematic. I
> would like to know how to calculate them in order to demonstrate that
> strange things happen when you use them (for a course for example). I
> know you can use drop1(lm(), test="F") but for an lm(y~A+B+A:B), type
> III SSQs are only calculated for the A:B interaction. If I specify
> lm(y~A+B), it calculates type II SSQ for the main effects (as type III
> only differs from type II if interactions are included). Thus, this
> approach doesn't help.
>
> Another approach is the Anova(, type="III") function within the
> library(car). But somehow it produces strange results. Somebody told me
> I have to change the contrast settings using
> options(contrasts=c("contr.helmert", "contr.poly"))
>  But I had the impression that my results are still impossible.
> Are the calculations dependent from the version of R I use? I am
> currently using R2.1.1
>
> The only thing that seems to work is a trick: Specify a separate column
> AB that codes a new variable for the interaction of A:B. Now you can fit
> A,B, and AB (as 3 main effects) in 3 different sequential models with
> each one of them in the end once. For the term in the end you then get
> type III SSQ which seem to be correct.
>
> Cheers
> Steffi

-- 
Michael W. Sears, Ph.D.
Postdoctoral Fellow
Department of Biology/MS 314
University of Nevada, Reno
Reno, NV 89557

Assistant Professor
Department of Zoology
Southern Illinois University
Carbondale, IL 62901

phone: 775.784.8008
cell:  775.232.3520
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        http://www.science.siu.edu/zoology/sears

"Natural selection is a mechanism for generating 
 an exceedingly high degree of improbability."
                        -Sir Ronald Fisher (1890-1962)