From: Spencer Graves <spencer.graves_at_pdf.com>

Date: Mon 16 Jan 2006 - 13:40:40 EST

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 Mon Jan 16 13:48:34 2006

Date: Mon 16 Jan 2006 - 13:40:40 EST

How about this:

> exp(coef(model3$modelStruct$varStruct)["const"])

const

0.6551298

Does that answer the question about not understanding the connection between summary(model3) and coef(model3$modelStruct$varStruct)["const"]?

Regarding the question about R not being able to find 'coef.varFunc',
I tried the following:

> methods("coef")

<snip>

coef.varFunc*

[37] coef.varIdent* coef.varPower*

Non-visible functions are asterisked

Since "coef.varFunc" is "asterisked", I tried 'getAnywhere(coef.varFunc)'. I walked through this code line by line, until I found the following:

> (val <- as.vector(model3$modelStruct$varStruct)) Variance function structure of class varConstPower representing

const power

0.6551298 0.8913665

Answer the questions? spencer graves

Rand, Hugh wrote:

> I have been using gnls with the weights argument (and varConstPower) to

*> specify a variance structure for curve fits. In attempting to extract the
**> parameters for the variance model I am seeing results I don't understand.
**> When I simply display the model (or use "summary" on the model), I get what
**> seem like reasonable values for both "power" and "const". When I actually
**> try to extract the values, I get the same number for the "power", but a
**> different (and less sensible) value for "const".
**>
**> The simplest example I can come up with that shows the problem is as
**> follows:
**>
**>
**> #Set up data
**>
**> x = c(0,0,0,1,1,1,2,2,2,3,3,3,4,4,4,5,5,5,6,6,6,7,7,7);
**> y0 = c(.1,.1,.1, .5,.5,.5, 1,1,1, 2,2,2, 4,4,4, 7,7,7, 9,9,9, 10,10,10);
**> yp = c(0,.03,.05, 0,.05,.01, 0,.07,.03, .1,0,.2, .2,.1,0, .3,0,.1,
**> 0,.3,.4, .3,.5,0);
**> y = y0 + 4*yp;
**> data = data.frame(x=x,y=y);
**>
**> #Run model
**>
**> library(nlme)
**> model3 = try(gnls(y ~
**> SSfpl(x,A,B,xmid,scal),data=data,weights=varConstPower(const=1,power=0,form=
**> ~fitted(.))));
**>
**> #Examine results
**>
**> model3; #const = .6551298, power =
**> .8913665
**> summary(model3); #const = .6551298, power =
**> .8913665
**>
**> coef(model3$modelStruct$varStruct)["power"]; # power =
**> .8913665
**> coef(model3$modelStruct$varStruct)["const"]; #const = -0.4229219
**> coef.varFunc(model3$modelStruct$varStruct); #R can't seem to find this
**> function, Splus can
**>
**>
**> Any advice on what I am doing wrong would be appreciated.
**>
**> Hugh Rand
**> Senior Scientist
**> Amgen
**> rand@amgen.com
**>
**> ______________________________________________
**> 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
*

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 Mon Jan 16 13:48:34 2006

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