Stata The Stata listserver
[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

st: Re: tobit and marginal effects


From   "Marina.Balboa" <Marina.Balboa@ua.es>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Re: tobit and marginal effects
Date   Mon, 4 Aug 2003 13:59:53 +0200

Thank you Scott. You are right, I think that is the answer!

----- Original Message -----
From: "Scott Merryman" <smerryman@kc.rr.com>
To: <statalist@hsphsun2.harvard.edu>; "Marina.Balboa" <Marina.Balboa@ua.es>
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:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



© Copyright 1996–2014 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index