[R-sig-ME] VarIdent, extracting delta value

[Afshartous, David]

All, 

When using varIdent to specify different residual error variance per strata,
one obtains the ratio between the standard deviations of the strata (see P&B
p.211), and this is included in the summary output.  However, perhaps I'm
missing something extremely basic but I don't see where this is within
str((summary(m1)) for a given model and hence cannot extract it. A
workaround is below, but if anyone knows the direct method please advise.

Cheers,
David


Library("nlme")
 m1 = lme(distance ~ age, random = ~ 1 | Subject, weights = varIdent(form =
~ 1 | Sex), data = Orthodont)
summary(m1)
## shows that delta = 0.4533690
##
Variance function:
 Structure: Different standard deviations per stratum
 Formula: ~1 | Sex 
 Parameter estimates:
     Male    Female
1.0000000 0.4533690

## 
str(summary(m1)) 

## workaround:
 1/attr(m1$modelStruct[[2]], "weights")[100]
   Female 
0.4533690 





######################################################
> sessionInfo()
R version 2.9.1 (2009-06-26)
i386-apple-darwin8.11.1

locale:
en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] nlme_3.1-92     lattice_0.17-25

loaded via a namespace (and not attached):
[1] grid_2.9.1         lme4_0.999375-31   Matrix_0.999375-29
>