Date:         Tue, 13 May 2008 15:43:23 -0500
Reply-To:     Anthony Babinec <tbabinec@sbcglobal.net>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Anthony Babinec <tbabinec@sbcglobal.net>
Subject:      Re: interaction in multinomial logistic
In-Reply-To:  <C9563A8511C9BB488B41E541ACC749F95E85C5A484@PITT-EXCH-07.univ.pitt.edu>
Content-Type: text/plain; charset="US-ASCII"

On the face of it, A should have one redundant parameter
and the A*B interaction should have one redundant parameter.
This comes about because of NOMREG's "set to 0" parameterization
of categorical variables. Double-check the messages to see
whether this is not in fact the case. Just speculating - Instead
of a stepwise method, try enter. If the A*B interaction is in
the model, you usually want the respective main effects in,
and this is not guaranteed with stepwise. Also, re-write the MODEL
statement to list A*B last.

-----Original Message-----
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of
Zdaniuk, Bozena
Sent: Tuesday, May 13, 2008 12:16 PM
To: SPSSX-L@LISTSERV.UGA.EDU
Subject: interaction in multinomial logistic

Hello, I am running a multinomial logistic with factor predictor A (three
categories), continuous predictor B and an interaction term A*B. (syntax
below).
For some reason, the results do not show the main effect for B ("this
parameter is set to zero because it is redundant").
I have hard time understanding why it is redundant. When running similar
model in binary logistic regression, I do get all the main effects. Anybody
willing to share some wisdom on that?
thanks so much.
Bozena

NOMREG
  grouping  (BASE=FIRST ORDER=ASCENDING) BY A WITH B C D
  /CRITERIA CIN(95) DELTA(0) MXITER(100) MXSTEP(5) CHKSEP(20) LCONVERGE(0)
  PCONVERGE(0.000001) SINGULAR(0.00000001)
  /MODEL= A*B A B C D
  /STEPWISE = PIN(.05) POUT(0.1) MINEFFECT(0) RULE(SINGLE) ENTRYMETHOD(LR)
  REMOVALMETHOD(LR)
  /INTERCEPT =INCLUDE
  /PRINT = PARAMETER SUMMARY LRT CPS STEP MFI .


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