Re: [R] Mixed Model notation SAS -> R

From: Spencer Graves <spencer.graves_at_pdf.com>
Date: Thu 18 May 2006 - 12:04:17 EST

          First, an almost trivial observation: The model you specified for lme could more succinctly be specified as follows:

          y ~ (a + b + c)^3

This expression implies the second and third order interactions. It does NOT imply squared terms in case any two of "a", "b", or "c" are numeric.

          To your question: Do you get the same result when you omit the "correlation" argument?

          Also, PLEASE do read the posting guide! "www.R-project.org/posting-guide.html". I very much appreciate you telling us you have Pinheiro and Bates. If you could have also included a simple, self-contained example, I might have been able to provide a more useful reply with less effort than what I've expended in writing these few lines.

	  Ich hofe dass diese weinige Woerter koenen Sie hilfen.
	  Spencer Graves

Jörg Trojan wrote:
> Dear experts,
>
> I'm trying to transfer a mixed model developed in SAS to R. This it what
> it looks like in SAS:
>
> proc mixed method=ml;
> class a b c subj;
> model y = a|b|c;
> repeated /subject=subj type=ar(1);
>
> I tried something like this in R:
>
> mixed <- lme(y ~ a + b + c + a*b + a*c + b*c + a*b*c,
> random = ~ 1 | subj,
> correlation = corAR1(form = ~ 1 | subj)
> na.action = na.omit, method = "ML")
>
> When I do an anova(mixed) the denomniator DFs do not compare to the ones
> SAS uses in calculating Type III results, In R a common, quite high
> (close to the total number of observations) denominator DF value is used
> for all effects, while SAS seems to calculate the DFs directly from the
> number of class categories. So obviously I have to specify my random
> effects in R differently, but I don't know how...
>
> Any hint on what I'm doing wrong is very much appreciated.
>
> Thanks,
> Jörg
>
> P.S. I have the Mixed Model book by Pinheiro and Bates here with me in
> case you can direct me to a specific section explaining my problem.
>
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R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Thu May 18 12:09:27 2006

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