Hello folkes,
I have a question concerning a 'simple' logistic regression model
with two dummy coded effects (one with 4 categories in 3 dummies and
the other with 2 categories in 1 dummy) and the interaction of these
two variables. The full model is comprised of several other variables
but of interest are these two effects and their interaction.
e.g. gender (male, female) vs. group (A,B,C,D)
==> gender=0 for male and gender=1 for female
==> group is split up in 3 variables GRP.B, GRP.C, GRP.D indicating
the group a person belongs to: GRP.B=1 if so and GRP.B=0 otherwise.
(This coding sets group A as a reference group)
==> interactions are IAGRP.B=GENDER*GRP.B
IAGRP.C=GENDER*GRP.C
IAGRP.D=GENDER*GRP.D
the code to do the logistic regression is (using the DESIGN library
but the problem is the same for glm() [pentiumII, PC, WIN95, S+ 3.3])
lrm(DEP ~ GENDER + GRP.B + GRP.C + GRP.C +
IAGRP.B + IAGRP.C + IAGRP.D, data=data)
questions 1)
Am I right, that in such a model there is no main effect of
'group'?
How do I interpret the coefficient of GENDER in the presence
of the interactions? From my point of view this is the diffence of females
compared to males in GROUP A (reference group), because all other
variables
are zero - that is: this cannot be interpreted as a usual main effect.
Is there a way to get an estimate of the 'main effect' of GENDER or is
it not possible because of the dummy coding and the interactions with
another dummy-coded effect?
What about an interaction between a dummy and a interval scaled variable?
questions 2)
The data do not necessarily suggest that group A should be the reference
any other could do so as well. But of course the coefficients of the
effects do change depending on which category of GRP.x is coded as the
reference
group.
Now I do have concerns about the arbitrariness of the results depending
on the reference group. That is: the statistical significance changes
depending on the reference group but the actual estimates (which you get)
from the regression equation) don't.
Do I see anything wrong here? Is this problem discussed somewhere to
get more information about it? How can I report such an analysis properly?
thanks for any comments - I will put a summary on the list.
yours, Matthias
----------------------------------------
Matthias Richard (doctoral candidate)
Center for Research on Psychotherapy
Christian-Belser-Str. 79a
70593 Suttgart
Germany
email: richard@psyres-stuttgart.de
telephone: ++49-711-6781-408
http://www.psyres-stuttgart.de
COST Action B6:
'Efficient Psychotherapy of Eating Disorders'
(COoperation of Science and Technology in the European Union)
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