reference category for factor in regression

Hi Thierry,

Thanks for your quick answer. The problem is not so much the LABOUR
variable, however, but the AGE variable, which consists of about 5
categories for which I do indeed not create separate dummy variables.
But R does not behave as expected when deciding on which dummy to use
as reference category ...

Jos

On Mon, Jan 19, 2009 at 2:37 PM, ONKELINX, Thierry
<Thierry.ONKELINX at inbo.be> wrote:

Dear Jos,

In R you don't need to create you own dummy variables. Just create a
factor variable LABOUR (with two levels) and rerun your model. Then you
should be able to calculate all coefficients.

HTH,

Thierry

------------------------------------------------------------------------
----
ir. Thierry Onkelinx
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and Forest
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tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

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than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
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-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Jos Elkink
Verzonden: maandag 19 januari 2009 15:16
Aan: r-help at r-project.org
Onderwerp: [R] reference category for factor in regression

Hi all,

I am struggling with a strange issue in R that I have not encountered
before and I am not sure how to resolve this.

The model looks like this, with all irrelevant variables left out:

LABOUR - a dummy variable
NONLABOUR = 1 - LABOUR
AGE - a categorical variable / factor
VOTE - a dummy variable

glm(VOTE ~ 0 + LABOUR + NONLABOUR + LABOUR : AGE + NONLABOUR : AGE,
family=binomial(link="logit"))

In other words, a standard interaction model, but I want to know the
intercepts and coefficients for each of the two cases (LABOUR and
NONLABOUR), instead of getting coefficients for the differences as in
a normal interaction model.

But the strange thing is, for the two occurances of the AGE variable,
it makes a different choice as to which AGE category to leave out of
the regression. The cross-table of AGE with LABOUR does not have empty
cells.

Anyone any idea what might be going wrong? Or what I could do about
this?

Thanks in advance for any help!

Regards,

Jos

--
Johan A. Elkink
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School of Politics and International Relations & CHS Graduate School
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