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st: RE: RE: -mfx- question & beta distribution


From   "Stephen P Jenkins" <stephenj@essex.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: -mfx- question & beta distribution
Date   Wed, 26 May 2004 16:05:44 +0100

Further to Nick's response: Nick's and my -betafit- fits a 2-parameter
Beta distribution where the parameters can depend on covariates.  My
quick glance suggests that Jack Buckley's -mlbeta- fits a 2-parameter
Beta distribution as well, though I am not sure about the relationships
between our parameterisation and his. [We referred to our parameters as
shape parameters; Jack splits his covariates into two groups, one for
mean (?) and one set in a variance() option.]

Regarding the original Qn about marginal effects, it would easier to
know what statistic that Jack wanted to calculate the marginal effect
of.  For most of these parametric distributions, summary statistics like
the mean or the variance (and pdf and cdf) are functions of the
parameters (and thence of covariates and coeff. estimates). These
formulae can be looked up in a book (or worked out).  Given them,
-nlcom- and -predictnl- are very useful commands for deriving the
summary statistics at different covariate values, and one could also use
them to derive marginal effects.  I don't think -mfx- is the place to
start.  (If only because we don't have a regression model here in the
way that I think that -mfx- assumes one has, in order to do its stuff
correctly.)

Stephen
-------------------------------------------------------------
Professor Stephen P. Jenkins <stephenj@essex.ac.uk>
Institute for Social and Economic Research
University of Essex, Colchester CO4 3SQ, U.K.
Tel: +44 1206 873374.  Fax: +44 1206 873151.
http://www.iser.essex.ac.uk   


> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu 
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> Sent: 26 May 2004 14:59
> To: statalist@hsphsun2.harvard.edu
> Subject: st: RE: -mfx- question
> 
> 
> Stephen Jenkins and I wrote a program called -betafit- 
> on SSC which I believe to be compatible with -mfx-. 
> Depending precisely what is wanted, that appears to be 
> an alternative. 
> 
> Nick 
> n.j.cox@durham.ac.uk 
> 
> Jack Buckley
> 
> > I wrote an ado file a while back called -mlbeta- that
> > estimates ML models for beta-distributed dependent variables 
> > (net from http://www2.bc.edu/~bucklesj). A user just asked me 
> > if the command supports -mfx-, which it does not (in that 
> > -mfx- does not return proper marginal effects). Can anyone 
> > give me a hint or two about what I would need to do? Kit Baum 
> > here at BC suggested that I might need to set-up the command 
> > so that -predict- would work properly first?
> 
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