# st: Re: tobit and marginal effects

 From "Marina.Balboa" To 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
>
> 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
>
> . 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
>
>
>
>
>
>
> *
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```