Re: [Impute] RE: Impute Digest, Vol 47, Issue 1

[Rod Little]

David, the sequential regression program IVEware allows for "mixed" 
variable types like the one you describe. I think it multiply imputes 
using a two stage model, for presence/absence and then amount given 
presence. Rod

On Mon, 7 Sep 2009, Gregorich, Steven wrote:

Hi, David.

It sounds like you are talking about a zero-inflated model, e.g., a 
zero-inflated Poisson (ZIP) or zero-inflated negative
binomial (ZINB). You could also fit a zero-inflated normal model (ZIN; Joe 
Schafer and a colleague wrote a paper
about that model in the late 1990's; they called it a 'two-part model'). I 
never studied the ZIN model closely, but
presumably the normal part of the model would allow negative predicted values, 
which I would not be happy with
in your application.

You can fit a 2-level multilevel ZIP, ZIN, ZINB, etc in PROC NLMIXED and you 
can probably find related SAS
code in the SAS-L archives; especially posts by Dale McLerran (sp?).

HTH

Steve
________________________________________

Message: 1
Date: Fri, 4 Sep 2009 14:25:33 -0400
From: David Judkins <judki...@westat.com>
Subject: [Impute] Robustness of Multi-Level Modeling Software
To: "impute@lists.utsouthwestern.edu"
       <impute@lists.utsouthwestern.edu>
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       <57e6162fdf3df442a02e784426c9c35c0b94e...@ex-cms01.westat.com>
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This is not an imputation question, but I don't know of a list serve for 
complex modeling questions.  Maybe one of you will be able to help.

Consider a mixed binary-normal distribution that results in a large point mass 
on the edge of an otherwise more-or-less normal distribution.  An example is 
number of alcoholic drinks per day.  Cigarettes per day is another example. Or 
the number of questions reading questions answered correctly on a sample that 
contains a large number of children who can't read at all.  The child reading 
example is my real concern because the children come grouped by school.

Anyone know of robustness studies of MLwin, HLM, Mixed, MPLUS, et cetera to 
this radical departure from normality?  I have heard it asserted that 
school-level departures from normality are more of a concern than student-level 
departures, but is this too much of a departure?


David Judkins
Senior Statistician
Westat
1650 Research Boulevard
Rockville, MD 20850
(301) 315-5970
davidjudk...@westat.com
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___________________________________________________________________________________
Roderick Little
Professor and Chair, Department of Biostatistics
U-M School of Public Health                 Tel (734) 936 1003
M4208 SPH II                                Fax (734) 763 2215
1420 Washington Hgts                        email rlit...@umich.edu
Ann Arbor, MI 48109-2029             http://www.sph.umich.edu/~rlittle/

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