# Re: st: RE: Tobit coefficients

 From May Boggess <[email protected]> To [email protected] Subject Re: st: RE: Tobit coefficients Date 03 Jan 2005 16:57:24 -0600

```On Monday, Lisa wrote:

>  For the following command:
>
>  tobit facil4p \$demo5, ll(0);
>  mfx compute,  predict(e(0,.));
>
>  does this give marginal effects in terms of
>
>  1) E(y|x,y>0)
>
>  OR
>
>  2) E(y|x) = P(y>0) * E(y|x,y>0)
>
>  If it is (1) (which I am pretty sure that it is) could someone please
> tell me how to specify (2) in STATA.
>

It gives (1). Here is an example that shows how the various predictions
available after -tobit- are calculated:

clear
sysuse auto
replace mpg=mpg-20
tobit mpg len, ll(0)
predict xb,xb
predict y, ystar(0,.)  /*  Unconditional Expected Value */
predict e, e(0,.)      /*  Conditional on being Uncensored */
predict p, pr(0,.)     /*  Probability Uncensored */

gen myy = p*e
sum y myy

gen mye = xb + _b[_se]*normden((0-xb)/_b[_se])/p
sum e mye

gen myp = norm(-((0-xb)/_b[_se]))
sum p myp

This shows that ystar is the function Lisa is looking for in (2). That
means she can calculate the marginal effect, and its standard error, as
follows:

mfx, predict(ystar(0,.))

--May
[email protected]

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```