[R-sig-ME] Incorrect std errors from nlmer?

r.millar at auckland.ac.nz r.millar at auckland.ac.nz
Tue Mar 31 10:11:30 CEST 2009

Hi All,

I just copied and paste the example nlmer code from the lmer help file
(see below), which fits the logistic to the orange tree data.
The point estimates of the fixed effects look correct, but the s.e.'s
are not (I've fitted the model using three other software). The s.e. of
Asym is way out (it should be about 15), as is the tree effect variance
(should be about 1000)??????


Russell Millar
U. Auckland

> (nm1 <- nlmer(circumference ~ SSlogis(age, Asym, xmid, scal) ~ Asym|Tree,
+               Orange, start = c(Asym = 200, xmid = 725, scal = 350)))
Nonlinear mixed model fit by the Laplace approximation
Formula: circumference ~ SSlogis(age, Asym, xmid, scal) ~ Asym | Tree
   Data: Orange
  AIC  BIC logLik deviance
 1901 1908 -945.3     1891
Random effects:
 Groups   Name Variance  Std.Dev.
 Tree     Asym 53985.920 232.349
 Residual         52.868   7.271
Number of obs: 35, groups: Tree, 5

Fixed effects:
     Estimate Std. Error t value
Asym   192.04     104.09   1.845
xmid   727.89      31.97  22.771
scal   347.97      24.42  14.252

Correlation of Fixed Effects:
     Asym  xmid
xmid 0.053
scal 0.050 0.763

> sessionInfo()
R version 2.8.1 (2008-12-22)

LC_COLLATE=English_New Zealand.1252;LC_CTYPE=English_New
Zealand.1252;LC_NUMERIC=C;LC_TIME=English_New Zealand.1252

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

other attached packages:
[1] lme4_0.999375-28   Matrix_0.999375-22 lattice_0.17-20

loaded via a namespace (and not attached):
[1] grid_2.8.1  tools_2.8.1

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