[R] robust regression and poisson errors

[Carlos Sperber]

We have done a multiple regression (glm) with non-orthogonal design and two 
correlated explanatory variables, and poisson errors, because the response 
variable is a count (number of species). The parameter estimates calculated 
were opposite to the tendency observed in the plot. As we excluded the most 
influencial point, the results changed radically, but as we took the second 
influential point the results changed again, to a third form.
Thefore I tried to use robust regression, with rlm. My questions are:
1 - how do I get the test of bias of the robust regression? In the S-Plus 
guide (p. 338), it is said that this test should be used to choose between 
the robust regression and the other one. Is this correct?
2 - when I run an anova, comparing the complete rlm model to a more simple 
model, I get no values for F nor P. How do I get these?
3 - may I use robust regression for count response variables, taking the 
log of the data?

Thank you all!

Carlos Sperber, Carla Galbiati & Carla Ribas

Carlos Frankl Sperber
Ecology - Orthoptera: Grylloidea

Departamento de Biologia Geral
Universidade Federal de Viçosa
3.570-000 - Viçosa - MG
Brazil

tel: +55 (31) 3899 2556
fax: +55 (31) 3899 2549
E-mail: sperber at ufv.br


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