Date:         Thu, 17 May 2012 10:49:50 -0700
Reply-To:     Bruce Weaver <bruce.weaver@hotmail.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Bruce Weaver <bruce.weaver@hotmail.com>
Subject:      Re: Multinomial Logistic Regression Interaction - graph the
              interaction odds ratios
In-Reply-To:  <4FB51966.8020401@uvm.edu>
Content-Type: text/plain; charset=us-ascii

Hello Susan.  I assume you are using NOMREG.  Have you taken a look at the
/TEST sub-command?  I think it might give you what you're after.  Here is a
very quick & dirty example I cobbled together.  Although one can perform
several contrasts with one /TEST sub-command (with semicolons separating the
various contrasts), I found I could not include labels for the contrasts
when I did that; hence the multiple /TEST sub-commands.  You might want to
direct the Contrast Results tables to a dataset via OMS to facilitate
grabbing the values you want for plotting etc.


GET FILE='C:\SPSSdata\1991 U.S. General Social Survey.sav'.

NOMREG race (BASE=LAST ORDER=ASCENDING) WITH age educ
  /MODEL age educ age*educ
  /STEPWISE=PIN(.05) POUT(0.1) MINEFFECT(0) RULE(SINGLE) ENTRYMETHOD(LR)
REMOVALMETHOD(LR)
  /INTERCEPT=INCLUDE
  /PRINT=PARAMETER SUMMARY LRT CPS STEP MFI.

graph histogram age.
graph histogram educ.

* Look at 4 levels of Age (20, 40, 60, 80) and 2 levels of Educ (10, 15).
* I.e., compare 10 and 15 years of education at each of the 4 ages.

NOMREG race (BASE=LAST ORDER=ASCENDING) WITH age educ
  /MODEL age educ age*educ
  /TEST "[1] Age=20, Educ=5"  ALL 1 20  5 100
  /TEST "[2] Age=20, Educ=10" ALL 1 20 10 200
  /TEST "[3] Age=40, Educ=5"  ALL 1 40  5 200
  /TEST "[4] Age=40, Educ=10" ALL 1 40 10 400
  /TEST "[5] Age=60, Educ=5"  ALL 1 60  5 300
  /TEST "[6] Age=60, Educ=10" ALL 1 60 10 600
  /TEST "[7] Age=80, Educ=5"  ALL 1 80  5 400
  /TEST "[8] Age=80, Educ=10" ALL 1 80 10 800
  /TEST "[2]-[1]" ALL 0 0 5 100
  /TEST "[4]-[3]" ALL 0 0 5 200
  /TEST "[6]-[5]" ALL 0 0 5 300
  /TEST "[8]-[7]" ALL 0 0 5 400
  /PRINT=PARAMETER SUMMARY LRT CPS STEP MFI
.

HTH.



SueRichardson wrote
>
> Hello,
>
> I haven't been able to find a question/solution quite like the problem I
> am having, so I'm creating a new thread.
> I have a reviewer of a paper who is requesting that I graph a
> significant interaction in my multinomial logistic regression
> differently than I have been.
>
> To briefly describe the regression, I have three levels of my DV, two
> continuous predictors (let's calls them A and B), and the interaction of
> those predictors (A*B).
>
> In previous drafts, I've followed the Jaccard (2001) approach by
> selecting points of interest for A and B, and used the regression
> equation to calculate predicted log odds. Because predicted log odds
> could be confusing, I converted these to predicted odds, and graphed
> those.
>
> While the reviewer acknowledged this was correct, she/he would rather
> see me graph the odds ratios instead of odds because of the possible
> misinterpretation of the odds as odds ratios.
>
> Where I am stuck is that I understand the "odds ratio" of the
> interaction to be the ratio of the odds ratios. So it's not really an
> odds ratio, itself.
>
> But I think the spirit of the reviewers question is that they want to see
> odds ratios, not odds.
>
> I've found guidance in this forum and elsewhere on the web about how to
> approach the problem of graphing interactions by manually calculating
> odds ratios when the variables are dichotomous to begin with, but not
> when the variables are continuous to begin with.
>
> What would work best to describe my interaction in terms of my
> hypotheses is to look at 4 levels of A (1,2,3,4) at 2 levels of B (+/-
> 1SD).
>
> So my question is, how do I calculate the odds ratios for different
> combinations of points of interest for my continuous variables?
>
> Can I use the odds ratios from the model I have and take the same
> approach as is used for categorical predictor interactions? Do I need to
> rerun the model dichotomizing the continuous predictors and their
> interaction? Or is there some other step that I am missing?
>
> Thanks for any guidance!
>
> Susan
>
> --
> Susan E.V. Richardson
> Postdoctoral Associate
> Person Environment Zone Project Manager
> 106 John Dewey Hall
> Psychology Department
> University of Vermont
> Burlington, VT 05405
> 1-866-532-7183
> susan.richardson@
> pez@
>


-----
--
Bruce Weaver
bweaver@lakeheadu.ca
http://sites.google.com/a/lakeheadu.ca/bweaver/

"When all else fails, RTFM."

NOTE: My Hotmail account is not monitored regularly.
To send me an e-mail, please use the address shown above.

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