[R] GLM fitting in R and Statistica
a.kloch at uj.edu.pl
Wed Apr 9 20:08:55 CEST 2008
I have a problem concerning discrepances between R (which I use) and
Statistica (which uses my supervisor). I can't say what is the origin
of these differences but unfortunately my supervisor doesn't know that
Our response variable is number (or presence/absence) of parasites in
rodents and explanatory variables are presence/absence of several
alleles. The rodents were sampled in three sites and the sites differ
in parasite frequency so we decided to include "site" as a factor.
The problem concerns calculations of factor variable. In Statistica
output there is only one term, just "site", and in R there are two
contrats "site A", "site B". I realized that I can obtain similar
results in Statistica clicking on "Estimate" button instead of "1LR"
(what does my supervisor which gives only log-likelihood and p of
variables but doesn't estimate parameter).
But there is still one problem I can't explain. When we fit
interaction terms (site x allele1, site x allele2 and so on) the
results are completely different. I tried several different contrasts
in R, such as contr.SAS, contr.treatment etc but I couldn't get
nothing similar to Statistica output.
Have anybody any idea how to deal with that? Or how to explain why R
results are different (and hopefully better)? I tried to argue that I
did everything according to Crawley's R Book so probably the models
are constructed correctly but my supervisor wasn't convinced...
Looking forward any suggestions,
Instytute of Environmental Sciences, Jagielonian University
ul. Gronostajowa 7, 30-387 Krakow, Poland, tel. (12) 664 51 51
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