st: Re: statalist-digest V4 #3224

[Tim Hale <timhale@uab.edu>]

Scott,

Actually, I haven't thought of this type of measure to be a count  
variable. Originally we used OLS but a reviewer suggested that we  
should transform the psychological distress score to correct for skew  
and if the skew is not corrected then we should try nbreg or zinb (if  
the vuong test shows a zero-inflated model to fit best). I tried  
correcting for the skew but sktest, swilk, and sfrancia indicate the  
distribution remains skewed. So, following the reviewers suggestion,  
we are trying zinb.

I have found a few papers where others have treated the CES-D as a  
count variable. We are using a very similar measure, the K6  
psychological distress scale.

Do you have any recommendations about how to handle the reviewers  
suggestions? Perhaps it is still appropriate to use the log- 
psychological distress rather than treating psychological distress as  
a count variable? The results between using OLS and the natural log of  
psychological distress are very similar to nbreg.

Thank you for help,
Tim

On Oct 25, 2008, at 1:33 AM, statalist-digest wrote:

> Date: Fri, 24 Oct 2008 07:23:58 -0700 (PDT)
> From: SR Millis <srmillis@yahoo.com>
> Subject: Re: st: suggested references about the variables to include  
> in zero-inflated portion of zinb?
>
> Tim,
>
> What is your basis for assuming that your response variable  
> (psychological distress) is a count variable?
>
>
>
> Scott R Millis, PhD, MEd, ABPP (CN,CL,RP), CStat
> Professor & Director of Research
> Dept of Physical Medicine & Rehabilitation
> Wayne State University School of Medicine
> 261 Mack Blvd
> Detroit, MI 48201
> Email:  smillis@med.wayne.edu
> Tel: 313-993-8085
> Fax: 313-966-7682
>
>
> - --- On Fri, 10/24/08, Tim Hale <timhale@uab.edu> wrote:
>
> > From: Tim Hale <timhale@uab.edu>
> > Subject: st: suggested references about the variables to include in  
> > zero-inflated portion of zinb?
> > To: statalist@hsphsun2.harvard.edu
> > Date: Friday, October 24, 2008, 1:18 AM
> > I am using zinb to estimate level of psychological distress
> > (scores
> > range from 0-24) using various demographic variables and
> > measures of
> > use of the Internet. I've used -countfit- to compare
> > various count
> > models and the results support zinb as the best fitting
> > model.
> >
> > I am uncertain, however, about how to justify the variables
> > that I
> > include in the zero-inflated part of the model. I've
> > read journal
> > articles that have used zinb, read the book by Freese and
> > Long, and
> > searched the Internet and Statalist but I have not been
> > able to find
> > any detailed recommendations or procedures. Can anyone
> > suggest any
> > other sources (books or journals) that provide an
> > explanation or a
> > good example of this process?
> >
> > Ideally I would like to find a good source that I can cite
> > in the
> > paper -- but I appreciate any suggestions about this you
> > might have.
> >
> > Thanks for you help,
> > Tim

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