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RE: st: RE: Fractional Logit


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: Fractional Logit
Date   Sun, 30 Oct 2005 21:54:41 -0000

You misunderstand my previous posts. I
was at some pains to emphasise that 
it is almost impossible, at least 
for me, to specify what is appropriate 
for your case on the information you provide. 
I did not "recommend" -betafit-. Clive 
Nicholas mentioned -mlbeta-. I just pointed
out that -betafit- is in the same territory. 

More important, the -alpha()- and -beta()-
options of -betafit- are explained in its 
help file. Again, the appropriate ways 
to include arguments for these options 
would presumably be some combination 
of theory and inspection of the data... 

Nick 
n.j.cox@durham.ac.uk 

Rijo John
 
> Thanking you for all your suggestions. Since my dependent variable is
> certainly a proportion between 0 and 1 (and not a binary with only 0,1
> values) now I am faced with three options.
> 
> 1) fractional logit model as in glm command (Where I am still not sure
>    which family link combination to be used)
> 2) mlbeta as suggested by clive
> 3) betafit as suggested by Nick
> 
> Nick, could you please explain the portion "alphavar(varlist1)
> betavar(varlist2)" in betafit command? Is there any standard way of
> deciding which are the variables to be included in the varlist1 and
> varlist2 for alphavar and betavar respectively?
 
> On Sun, 30 Oct 2005, Nick Cox wrote:
> 
> <If that is a possibility, so also is the -betafit-
> <of Stephen Jenkins et alius, downloadable from SSC.
> <
> <Nick
> <n.j.cox@durham.ac.uk
> <
> <Clive Nicholas
> <
> <> Rijo John replied to Nick Cox:
> <>
> <> > Thanks Nick. I am trying to estimate the impact of spending on a
> <> > particular commodity (say X) on the purchase of other goods
> <> and services.
> <> > So I have the share of, say commodity Y (Its share in
> <> household budget)
> <> > as the dependent variable and the expenditure on 
> commodity X along
> <> with
> <> > various control variables for household as explanatory
> <> variables. This
> <> > can be a standrad OLS estimation. But since the depended
> <> variable is a
> <> > fraction and  is bound between 0 and 1 we go for fractional
> <> logit model.
> <> >
> <> > I hope this clarifies the problem. Given this I would like
> <> to know what
> <> > would be the right choice of family and link in GLM.
> <> >
> <> > "family(gaussian) link(logit)" or "family(binomial)
> <> link(logit)" or some
> <> > other combination?
> <> >
> <> > in "family(binomial) link(logit)" is it tru that stata
> <> takes all values
> <> > other than 0 in the dependent variable as 1. If thats the
> <> case it does
> <> > not take the budget share (absolute amount) information
> <> into account. It
> <> > will only see if the budget share is absent or not.
> <>
> <> Since your dependent variable is a fractional, or 'compositional',
> <> variable ranging continuously from 0 to 1, another
> <> alternative is to use
> <> Sean 'Jack' Buckley's -mlbeta-. This allows maximum
> <> likelihood estimation
> <> with Beta-distributed dependent variables, as well as
> <> modelling separate
> <> equations for both a mean and dispersion effect. This 
> routine is _not_
> <> downloadable from SSC, so type:
> <>
> <> . net from http://www2.bc.edu/~bucklesj
> <>
> <> and then click on -mlbeta-. Hope that helps.

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