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Re: st: RE: How to avoid negative fitted values


From   Laurie Molina <molinalaurie@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: How to avoid negative fitted values
Date   Fri, 20 Aug 2010 22:32:08 -0500

Thank you Benjamin,
I was thinking on your first option... isn't it a Poisson regression?
My data is not count data and i have large observations (the mean is 300,000).
Would it be ok to use the poisson regression? Or is there any other
distribution that i could use with glm and a log link?

Again, thank you very much...



On Fri, Aug 20, 2010 at 10:18 PM, Craig, Benjamin M.
<Benjamin.Craig@moffitt.org> wrote:
> There are two ways that make sense to me, but their use depends on your theory.
>
> 1. glm with a log link will produce positive estimates (see work of manning and mullahy)
>
> 2. Inequality constraints on the parameter space (much more difficult)
>
> It sound like you are using a linear model on skewed data, and you are overshooting the skew by assuming a symmetric distribution. Maybe if you assume a gamma distribution (see Basu) your predictions will get better. Again this is all dependent on your theory about the origin of the error. I am not in real estate, sorry.
>
> Ben
>
>
> Benjamin M. Craig, Ph.D. Assistant Member, Health Outcomes & Behavior Moffitt Cancer Center Associate Professor, Department of Economics University of South Florida Contact Information 12902 Magnolia Drive, MRC-CANCONT Tampa, FL 33612-9416 Phone: (813) 745-6710 Fax: (813) 745-6525 benjamin.craig@moffitt.org
>
> ________________________________
>
> From: owner-statalist@hsphsun2.harvard.edu on behalf of Laurie Molina
> Sent: Fri 8/20/2010 10:26 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: How to avoid negative fitted values
>
>
>
> Dear statalist,
>
> I am triying to imput rental prices for houses to a sample using some
> characteristics of the construction which are shared between my data
> and the data in another database in which rental prices are declared.
>
> When i use OLS i get negative fitted values both in the original
> database as in my database.
>
> How can ensure that the predicted values will be non negative?
>
> Thank you very much in advance.
>
> Laurie
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