Date: Fri, 17 Jun 2005 07:18:59 -0500
Reply-To: "Peck, Jon" <email@example.com>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: "Peck, Jon" <firstname.lastname@example.org>
Subject: Re: Unexpected singularities
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To expand on Hector's point, the singularity could be due to other predictor variables in the equation. Redundancy in a single dummy variable is the easiest to understand, but you have to consider the whole set of predictors.
Imagine that you have a categorical variable x1 with values a, b, c, but when x1 is c, the x2 value is a constant, d. Then including the dummy variables for a, b, c and for x2 will result in singularity: you cannot distinguish the effect of x1 being c from the effect of x2 being d.
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of Rayman Mohamed
Sent: Thursday, June 16, 2005 10:03 PM
Subject: [SPSSX-L] Unexpected singularities
I have been doing some multinomial logistic regressions.
When I use two particular categorical variables (in
appropriate dummy form -- see later) in my models I get the
"Unexpected singularities in the Hessian matrix are
encountered. This indicates that either some predictor
variables should be excluded or some categories should be
This implies that I have redundancy in my dummy variables (I
think). However, I have checked and rechecked but I cannot
see where I am going wrong. One of the categorical variables
for which I am getting this message is political
affiliation. I created three dummy variables (democrat,
republican, and independent/no affiliation) and entered two
into the regression model. (There are other categories that
I ignored when I created the dummy variables, e.g.,
the "don't knows".)
Could anyone think of another reason why I am getting this
message. Or do you think that perhaps I just looked at this
too much so I cannot see some rather obvious mistake in my
syntax to create the dummy variables?
Here is the syntax that I used to create the dummy variables
for political affiliation:
IF (q78a = 1) democrat_1 = 1 .
IF (q78a = 2 | q78a = 3 | q78a = 5) democrat_1 = 0 .
IF (q78a = 2) republican_1 = 1 .
IF (q78a = 1 | q78a = 3 | q78a = 5) republican_1 = 0 .
IF (q78a = 3 | q78a = 5) Ind_other_1 = 1 .
IF (q78a = 1 | q78a = 2) ind_other_1 = 0 .
Thanks for your suggestions,
Wayne State University
College of Urban, Labor, and Metropolitan Affairs
Department of Geography and Urban Planning
227 State Hall
5143 Cass Avenue
Detroit, MI 48202