Dear R users,

First, I apologize. This question is not code related, but I figured I
should ask it here given the large amount of questions pertaining to lme
and gls models that I see posted.

I am analyzing the effects of two variables (LDH and SI3) on total soil C.
The model that I am using for this analysis is a gls model, which is below:

m2<-gls(C~LDH + SI3, method = "ML", weights = vpDist)

What I want to know is, can anyone guide me to a reference where I can read
about, understand, and interpret the correlations given in the gls model
output for correlations among parameter estimates? I have read through
Pinheiro and Bates and Zuur (2009)'s books on mixed effects models (which
also cover gls models), and searched the web, but I have found very little
on this part of the model output.

I ask, because I want to know if I should be concerned by a high
correlation value between two parameters (e.g. between LDH and Intercept ~
0.92; see below). Zuur (2009) on page 138 gives an example of a lme model
with a high correlation value between an explanatory value and the
intercept and then suggests centering the explanatory value by subtracting
the mean of the explanatory value from all individual values for this term.
I, however, am unclear as to why one should do this, particularly if
someone is interested in generating a reliable estimate for the intercept.

Thanks again for any suggestions that anyone has for me.

The output for my model is:

> summary(m2)
Generalized least squares fit by maximum likelihood
  Model: C ~ LDH + SI3
  Data: NULL
       AIC      BIC    logLik
  440.6114 453.4332 -215.3057

Variance function:
 Structure: Power of variance covariate
 Formula: ~DistHyp
 Parameter estimates:
     power
-0.7507717

Coefficients:
                Value Std.Error   t-value p-value
(Intercept)  5.515769 2.3318502  2.365404  0.0201
LDH         -1.671476 0.5551217 -3.011007  0.0034
SI3          0.096973 0.0319332  3.036747  0.0031

 Correlation:
    (Intr) LDH
LDH -0.924
SI3 -0.161 -0.217

Standardized residuals:
        Min          Q1         Med          Q3         Max
-2.68192339 -0.67510193 -0.05998944  0.56286828  2.55012832

Residual standard error: 45.15951
Degrees of freedom: 96 total; 93 residual

-- 
Basil Iannone
University of Illinois at Chicago
Department of Biological Sciences (MC 066)
845 W. Taylor St.
Chicago, IL  60607-7060
Email: bianno2@uic.edu
Phone: 312-355-3231
Fax: 312-413-2435
http://www2.uic.edu/~bianno2

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