[R-sig-ME] gls error

[Benjamin Gillespie]

Morning all,

I wonder if anyone could shed some light on a problem I am receiving in R when I try to fit a model:

I'm attempting to follow the 'Protocol' as in Chapters 4 & 5 of Zuur et al 2009 for some data I have for a number of river sites, sampled once for macroinvertebrates. Each site has been graded into 1 of 3 groups dependent on it's characteristics. I want to find out whether the factor: "group" is significant.

I have a response variable: "simp" (simpsons diversity index) and a number of fixed factors that I would like to include in my model.

In R, this is the code I use:

f1=formula(simp~group+date+altitude+data_source+catchment_size+g1+g2+g3+g4+g5+g6+lc1+lc2+lc3+lc4+lc5)
s1.gls=gls(f1,data=env.sp)

Please note: g1...gX are %cover of geology types for each site and lc1...lcX are % land cover types for each site.

The following is the error I receive:

Error in glsEstimate(glsSt, control = glsEstControl) : computed "gls" fit is singular, rank 16

>From what I've read, it looks like I'm using too many explanatory factors. I have tested for colinearity between all factors, but none look like obvious candidates for removal.

What would you suggest one should do in this instance?

Many thanks in advance for your advice,
		
Ben Gillespie
Research Postgraduate