Gus...what about not doing a log transformation, but instead try to model
the heterogeneity (which is the reason, I assume, you do a transformation??) using varIdent?
Quite often observations from the control group have more variation.

See Chapter 5 in Pinheiro and Bates (2000) for an explanation how to do this.

Alain




Greetings,
I doubt this is a particularly interesting question for you mixed
model gurus, but here goes.  As you can see in the output below, I
have a model with twelve fixed effect parameters.  I am interested in
each of the "Treatment" vs. "Control" comparisons for each "site"(in
each fixed effect parameter name, these are specified by the text
immediately following "sitett"). To produce a 95% CI for such a
comparison I was advised to take two steps:

(1) Subtract the Control parameter estimate from the Treatment
parameter estimate for each site.
(2) Compute the SE for this comparison via:  sqrt( var(treatment) +
var(control) - 2*cov(treatmentt,control)).  To get these values I am
using the vcov matrix for the model.

When I move to log10-transformed data, I am thinking I should
backtransform the fixed effects and SE's before moving ahead  with the
Control-Treatment comparisons.  However, the calculations become more
problematic as ( var(treatment) + var(control) -
2*cov(treatmentt,control)) is consistently negative.  I am uncertain
on how to proceed here.  Any advice would be much appreciated.

Thank you,
Gus








-- 

Dr. Alain F. Zuur
First author of:

1. Analysing Ecological Data (2007).
Zuur, AF, Ieno, EN and Smith, GM. Springer. 680 p.
URL: www.springer.com/0-387-45967-7


2. Mixed effects models and extensions in ecology with R. (2009).
Zuur, AF, Ieno, EN, Walker, N, Saveliev, AA, and Smith, GM. Springer.
http://www.springer.com/life+sci/ecology/book/978-0-387-87457-9


3. A Beginner's Guide to R (2009).
Zuur, AF, Ieno, EN, Meesters, EHWG. Springer
http://www.springer.com/statistics/computational/book/978-0-387-93836-3


4. Zero Inflated Models and Generalized Linear Mixed Models with R. (2012) Zuur, Saveliev, Ieno.
http://www.highstat.com/book4.htm

Other books: http://www.highstat.com/books.htm


Statistical consultancy, courses, data analysis and software
Highland Statistics Ltd.
6 Laverock road
UK - AB41 6FN Newburgh
Tel: 0044 1358 788177
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