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st: Re: statalist-digest V4 #3224

From   Tim Hale <>
Subject   st: Re: statalist-digest V4 #3224
Date   Sat, 25 Oct 2008 17:46:30 -0500


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,

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

Date: Fri, 24 Oct 2008 07:23:58 -0700 (PDT)
From: SR Millis <>
Subject: Re: st: suggested references about the variables to include in zero-inflated portion of zinb?


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
Tel: 313-993-8085
Fax: 313-966-7682

- --- On Fri, 10/24/08, Tim Hale <> wrote:

From: Tim Hale <>
Subject: st: suggested references about the variables to include in zero-inflated portion of zinb?
Date: Friday, October 24, 2008, 1:18 AM
I am using zinb to estimate level of psychological distress
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

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,

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