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st: Tobit, negative predictesd values


From   "Solorzano Mosquera, Jenniffer" <jenniffers@iadb.org>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: Tobit, negative predictesd values
Date   Mon, 9 Nov 2009 17:59:03 -0500

Hi everyone,
I estimated a tobit model having intensity labor training as dependent variable and a group of firm characteristics which are presumpted as determinants of that intensity. I need to predict the estimated intensity to use it in a second stage model to know what is the impact in firm productivity. I'm using the whole sample, those firms offering training and those who are not. I'm censoring to the interval 0,1. However I obtain negative predicted variables from the tobit.

I have 0's for those firms who decided not to offer training, following a published paper by Zwick in 2002(Centre for European Economic Research). That way allows not to lose observations from the firms who have missing values in the intensity variable because they decided not to offer training. However I've been looking and I found that heavy censoring causes these kind of problems on predicted values and even worse when high proportion of
censored cases is the situation.

Any suggestion to obtain tobit predicted values non-negatives or is it can be interpreted as a expected result?

Thanks



Jenniffer Solorzano Mosquera
Labor Markets Division - Inter-American Development Bank
Washington, DC - USA
(202) 623-3659


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