Date: Thu, 4 Aug 2005 09:39:24 EDT
Reply-To: Marijaspss@aol.com
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Marija Norusis <Marijaspss@aol.com>
Subject: Re: Reference Category for Categorical Vars in NOMREG
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If you declare the dichotomous variable to be a covariate instead of a
factor, the original coding is maintained.
Marija Norusis
_www.norusis.com_ (http://www.norusis.com)
In a message dated 8/4/2005 8:28:33 A.M. Central Standard Time,
mscoppettone@IAGR.NET writes:
The lesson on the semantics of "reference category" aside, my point is the
following:
For a binary variable coded as 0 or 1, NOMREG is estimating the effect of
taking the value of 0 while setting the coefficient for level 1 equal to
0. In general, a "0" is used to indicate that the characteristic is not
present, and vice versa. Thus is would be helpful is one could get NOMREG
to estimate the effect of the variable taking the value 1 for convenience
purposes.
Clearly, a little simple arithmetic can be applied to the output to get
this, but it would be helpful if there were an option for this.
On Thu, 4 Aug 2005 00:29:49 -0300, Hector Maletta
<hmaletta@fibertel.com.ar> wrote:
>Marija,
>thanks for the clarification, but on the whole I found it slightly
confusing
>anyway. However, my point was chiefly to make a distinction between this
use
>of "reference category" and the use of the same name in other procedures
>such as Logistic or Cox Regression, when the conversion of categorical
>variables into dummies is concerned. In NOMREG the category in question is
>not omitted: it is the one for which the estimated effect is equivalent to
>the intercept.
>
>On the other hand, leaving to the user the option of designating another
>category for that role may be extremely nice, and it is indeed
regrettable,
>though harmless, that SPSS programmers had chosen not to leave that
option.
>
>Besides, even if that option is not implemented, it should be more clearly
>stated that alternative models are possible where the "intercept" category
>is one of the other categories in the factor, other than the last one, and
>provide the formulas to convert one model results into another.
>
>Hector
>
> _____
>
>From: Marijaspss@aol.com [mailto:Marijaspss@aol.com]
>Sent: Wednesday, August 03, 2005 11:41 PM
>To: hmaletta@fibertel.com.ar; SPSSX-L@LISTSERV.UGA.EDU
>Subject: Re: Reference Category for Categorical Vars in NOMREG
>
>
>
>
>On page 46 of the Advanced Statistical Procedures Companion it states,
>
>"The SPSS Multinomial Logistic Regression procedure treats the last
category
>of a categorical or factor variable as the reference category and sets its
>coefficient to 0. The variable male is coded 0 for no and 1 for yes, so
>males are the reference category."
>
>Marija Norusis
>www.norusis.com
>
>
>
>In a message dated 8/3/2005 5:30:22 P.M. Central Standard Time,
>hmaletta@fibertel.com.ar writes:
>
>By the way, in Marija Norusis' book (SPSS 13.0 Advanced Statistical
>Procedures Companion) I find the explanation of this matter rather
confusing
>(p.47). First, it is not clear in her example which gender is 0 and which
is
>1: in the text it seems that the variable (confusingly called MALE) is
zero
>for males, since she says "For males the estimated value is just the
>intercept, since the coefficient for the predictor variable is 0" whereas
in
>the accompanying table the category with B=0 is MALE=1. Secondly, the
table
>has a footnote regarding that zero, indicating that "The parameter is set
to
>zero because it is redundant", when in fact it is not exactly so: the true
>explanation IMHO is that the parameter is zero by construction: the model
>being tested establishes that the effect of being male is just the
>intercept, and the effect of being female is the intercept plus the effect
>of gender.
>
>
>
>
>
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