[R-sig-ME] level-dependent explanatory variables

[Julian PICHENOT]

Dear mixed models list,

I am trying to fit models with lme4 for a data set which has an  
unbalanced and hierarchical structure.
The goal is to model the presence/absence (0/1) of a frog species in  
ponds, considering potential explanatory variables measured at three  
levels (spatial scales).

The nested structure is as follow : 1516 ponds are nested within 134  
patches and these patches are nested within 24 landscapes.

There are 3 variables measured at each level :
Pond level : A1,B1,C1
Patch level : D2,E2,F2
Landscape level : G3,H3,I3.

This is the structure of the data set :
> str(msc)
'data.frame':   1516 obs. of  13 variables:
  $ y      : int  0 1 0 1 0 0 0 1 0 0 ...
  $ A1   : num  -0.758 -0.835 -0.835 -0.757 -0.757 ...
  $ B1   : num  -1.77 -1.77 -1.77 -1.80 -1.80 ...
  $ C1   : num  -0.262 -0.189 -0.189 -0.286 -0.286 ...
  $ D2  : num  0.869 0.869 0.869 0.869 0.869 ...
  $ E2: num  -2.49 -2.49 -2.49 -2.49 -2.49 ...
  $ F2   : num  -1.09 -1.09 -1.09 -1.09 -1.09 ...
  $ G3  : num  -0.327 -0.327 -0.327 -0.327 -0.327 ...
  $ H3  : num  -1.56 -1.56 -1.56 -1.56 -1.56 ...
  $ I3 : num  1.15 1.15 1.15 1.15 1.15 ...
  $ POND   : int  1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 ...
  $ PATCH  : int  1 1 1 1 1 1 1 1 1 1 ...
  $ LAND   : int  1 1 1 1 1 1 1 1 1 1 ...

Due to the fact that they are measured at a higher level than the  
pond, the values of D2,E2,F2 are repeated for each pond in a one  
particular patch and the values of G3,H3,I3 are repeated for each pond  
of each patch in one particular landscape.

Here is the code that I use :

> glmer(y~A1+B1+C1+D2+EP2+F2+G3+H3+I3+(1|LAND/PATCH),family=binomial,data)

But I find the results a bit strange. Only the variables measured at  
the pond level are significant and I have doubts about this.

Is there another way to fit a model that takes into account the  
spatial scale of each explanatory variable ?

I use the following packages :

> sessionInfo()
R version 2.7.0 (2008-04-22)
i386-pc-mingw32

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base
other attached packages:
[1] lme4_0.999375-22   Matrix_0.999375-10 lattice_0.17-6
loaded via a namespace (and not attached):
[1] grid_2.7.0

Thanks in advance for your help and answers.
Best regards.

Julian