[R] How to extract parameter estimates of variance function from lme fit

[Hoai Thu Thai]

Dear R-Users,

I have a question concerning extraction of parameter estimates of 
variance function from lme fit.

To fit my simulated data, we use varConstPower ( constant plus power 
variance function).
fm<-lme(UPDRS~time,data=data.simula,random=~time,method="ML",weights=varConstPower(fixed=list(power=1)))
I extract the results of this function by using the following codes:
y<-summary(fm)
x<-y$modelStruct$varStruct
 > x
Variance function structure of class varConstPower representing
        const        power
1.184535e-15 1.000000e+00

These are the constants and power that apppear in the summary(fm), and 
that I want to extract.
Hower I got different value of const when extracting from x:

 > x$const
     const
-34.36943

Does anyone know why there is such difference and how to extract the 
expected value (1.184535e-15) ?

Thanks in advance for your help.

Thu


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THAI Hoai Thu
INSERM U738 - Université Paris 7
16 rue Henri Huchard
75018 Paris, FRANCE
Tel: 01 57 27 75 39
Email: hoai-thu.thai at inserm.fr