Hi all,
I have a problem estimating predicted values that is
somehow related to the messages fro Lisa and May
below.
I estimated I tobit model and I want E(y|x,y>0)
conditional on vector x on its mean values. After
reading the messages below I understand that the
following command gives me what I want:
mfx compute, predict(e(0,.))
(It also gives me the marginal effects, but I don't
need them)
My problem is that mfx does not give me the Standard
Error of E(y|x,y>0). And I need this.
I have tried the prvalue command. Simply typing
"prvalue", with no options", it gives me the predicted
value and a confidence interval, which would be
enough. But what prvalue estimate is E(y|x), and not
E(y|x,y>0).
Does anyone know how to make prvalue estimate
E(y|x,y>0) after a Tobit model, rather than E(y|x)?
Or is there another way to do what I want?
Or am I getting it all wrong from the beginning and my
question makes no sense at all?!..
Thanks in advance,
Vitor
----- Original Message -----
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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