Thank you Scott. You are right, I think that is the answer!
----- Original Message -----
From: "Scott Merryman" <[email protected]>
To: <[email protected]>; "Marina.Balboa" <[email protected]>
Sent: Friday, August 01, 2003 7:00 PM
Subject: st: tobit and marginal effects
> Marina,
>
> I am sending this to the list as well - I'm not sure I'm correct and
others have
> additional insights.
>
> Marina asked me off-list why, when you request the marginal effects of the
> unconditional expected value following tobit, does Stata list in its
output "y =
> E[y*|y> ll]" where ll is the lower limit; y* = y if y > ll and y* = ll if
y <=
> ll; i.e. does Stata really give the unconditional marginal effects.
>
> Example:
>
> . use "C:\Stata8\auto.dta", clear
> (1978 Automobile Data)
>
> . replace price = 4000 if price <4000
> (11 real changes made)
>
> . qui xttobit price mpg, ll(4000) i(foreign)
>
> . mfx compute, nose predict(ys(4000,.))
>
> Marginal effects after xttobit
> y = E(price*|price>4000) (predict, ys(4000,.))
> = 6495.1777
> --------------------------------------------------------------------------
-----
> variable | dy/dx X
> ---------------------------------+----------------------------------------
-----
> mpg | -252.7986 21.2973
> --------------------------------------------------------------------------
-----
>
>
> I believe the unconditional expectation of y is
>
> E[y]= E[y|y>ll]*P(y>ll) + E[y|y<=ll)*P(y<ll)
>
> = E[y|y>ll]*P(y>ll) + 0*P(y<ll)
>
> = E[y|y>ll]*P(y>ll)
>
> Hence, the unconditional expectation uses information on the censoring
value.
>
> Hope this helps,
> Scott
>
>
>
>
>
>
> *
> * For searches and help try:
> * http://www.stata.com/support/faqs/res/findit.html
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
*
* For searches and help try:
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* http://www.ats.ucla.edu/stat/stata/
