From: K. Steinmann <Katharina.Steinmann_at_stud.unibas.ch>

Date: Mon 15 Aug 2005 - 23:39:54 EST

Date: Mon 15 Aug 2005 - 23:39:54 EST

Dear R-helpers,

I try to perform glm's with negative binomial distributed data.
So I use the MASS library and the commands:
model_1 = glm.nb(response ~ y1 + y2 + ...+ yi, data = data.frame)
and

predict(model_1, newdata = data.frame)

So far, I think everything should be ok.

But when I want to perform a glm with a subset of the data, I run into an error message as soon as I want to predict values, based on the new model. The problem seems to be the reduced number of levels of one of the factors yi ( a categorical factor) in the subset of the original data set.

On cran search I found some related hint, that the line "mf$drop.unused.levels <- TRUE " in the glm (or glm.nb) function could cause the problem.

Therefore I changed the line to "mf$drop.unused.levels <- FALSE ". Indeed the error message disappears and when I compare the prediction of model_1 with the prediction of the model, carried out with the full data set but with the changed glm.nb function, I get the same predicted numbers.

However, the change of glm.nb function was more of an intuitive action, and since I still consider myself as a beginner of R, I don't feel comfortable.

So my questions:

1. Is there an easier way to solve my problem?
2. Do I affect the glm.nb function seriously, by changing the line mentioned
above?

Thank you for your help,

Katharina

PS: I am working with R 2.0.0

PPS: Concrete error message:

"Error in model.frame.default(Terms, newdata, na.action = na.action, xlev =
object$xlevels) :

factor I(kanton) has new level(s) GE"

-- K. Steinmann Botanisches Institut Universität Basel CH-4056 Basel Switzerland Tel 0041 61 267 35 02 E-mail: Katharina.Steinmann@stud.unibas.ch ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.htmlReceived on Mon Aug 15 23:47:06 2005

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