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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