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Re: st: AW: Tobit, negative predictesd values


From   Austin Nichols <austinnichols@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: AW: Tobit, negative predictesd values
Date   Tue, 10 Nov 2009 11:00:33 -0500

If you want predictions in [0,1], a new model is probably in order:
http://www.stata.com/support/faqs/stat/logit.html

On Tue, Nov 10, 2009 at 10:44 AM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> In a different vein, but I think consistently with Maarten's suggestions:
>
> 1. You are fitting a hyperplane in some space. That could stay entirely above the origin but that's a tall order if many responses are at or very near zero. Perhaps your model could do with some (more?) curvature....
>
> 2. Otherwise put, if you believe that no predicted responses can plausibly be negative, this functional form won't ensure that.
>
> 3. Plot observed vs predicted and residual vs predicted to get some handles on where your model is misbehaving.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Maarten buis
>
> That is perfectly consistent with the logic behind the -tobit-
> model: It assumes that there is some latent variable (the ideal
> intensity of training), but this can only be realized when this
> ideal is larger than some cut-off point, in this case 0.
>
> The default for the predicted values are these ideal levels of
> training (the linear predictor). So, what you found is that
> some employers ideally would want to take training away from
> their employees. If you think that that is not a senisible
> interpretation then the -tobit- may not be the suitable model
> for your situation.
>
> ---  Solorzano Mosquera, Jenniffer wrote:
>
>> 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.
> <snip>
>> 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.
>

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