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Re: st: logY, Tobit and the prediction of Y


From   mathew.stalker@dotecon.com
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: logY, Tobit and the prediction of Y
Date   Wed, 23 Apr 2003 14:02:32 +0100






owner-statalist@hsphsun2.harvard.edu wrote on 23/04/2003 13:18:03:

> Hi,
>
> My initial model is:
>
> Y = a + b1x1 + controls + e
>
> where Y is expenditures on a commodity and x1 is income.
>
> Since there are a lot of zeroes, I use the Tobit apporach. However, since
> the log-linear model performed better than the linear, I use the former
> (Before the log transformation of Y, I follow convention and set zeroes
to
> 1.)
>
> Accordingly, the estimated Tobit model is:
>
> logY = a + b1x1 + controls + e
>
> The problem:
>
> I want to predict the value of Y (not logY) for certain values of income
> (and put it in a graph); that is, both the conditional Y (i.e. the Y
given
> that the threshold value of 0 was passed) and the unconditional (latent)
> value of Y.
>
> Does anyone know how to do this?
>
> Best regards,
>
> Christer Thrane
> Lillehammer University College
> Norway
>
>
>
>
>
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/

The prediction of Y from your model would simply be the exponential of the
predicted logY.

However, you should note that the log of zero is minus infinity, so in your
log model no observations where Y is zero will be included.  Is this really
what you want?

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



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