Date: Wed, 13 Oct 1999 16:23:15 +0100
Reply-To: "Daniel P. Mears" <dpmears@MAIL.LA.UTEXAS.EDU>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: "Daniel P. Mears" <dpmears@MAIL.LA.UTEXAS.EDU>
Subject: Deviation contrast in GLM
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I'm curious about what exactly the deviation contrast in GLM-Univariate
(and in the logit module) does. The SPSS online help indicates that the
deviation contrast provides information on whether EACH category of an
independent variable differs statistically from the grand/overall mean of
ALL of the categories of the independent variable. Thus, with a 5-category
independent variable, it provides information on whether the mean for
category 1 differs statistically from the grand/overall mean for categories
1-5, on whether the mean for category 2 differs statistically from the
grand/overall mean for categories 1-5, and so on.
There are two issues about which I am particularly confused. First, to
obtain the full set of contrasts (i.e., for each category of the
independent variable), you have to run the GLM-Univariate option twice,
once with the first category of the independent variable "omitted" and a
second time with the last category "omitted." The very nature of the
deviation contrast option would seem to suggest that "omitting" ANY
category makes no sense.
Second, how does this deviation contrast option "work"? For example, it
seems like those categories that contribute disproportionally more cases to
the grand/overall mean would consistently be more likely to be closer to
the grand/overall mean, and thus it seems likely that any derived statistic
would be suspect. (I speak here as a humble user of statistics, not as one
who is particularly knowledgeable in this area.)
Any comments or clarification would be appreciated.