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st: Re: Re: marginal effects


From   "Marina.Balboa" <Marina.Balboa@ua.es>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Re: Re: marginal effects
Date   Thu, 24 Jul 2003 10:20:55 +0200

Thank you Scott very much for your helpful explanation. Just one question,
please. What about the constant? If I want to report the results of

mfx compute, predict(ys(0,.))

Does not the model on the observed variable y have a constant, like the
model with the latent variable?

Or does it have a constant that I may infer like this (using your example):

> . xttobit price mpg, ll(4000) i(foreign) nolog
>
> Random-effects tobit regression                 Number of obs      =
74
> Group variable (i): foreign                     Number of groups   =
2
>
> Random effects u_i ~ Gaussian                   Obs per group: min =
22
>                                                                avg =
37.0
>                                                                max =
52
>
>                                                 Wald chi2(1)       =
25.39
> Log likelihood  = -597.93768                    Prob > chi2        =
0.0000
>
> --------------------------------------------------------------------------
----
>        price |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
> -------------+------------------------------------------------------------
----
>          mpg |  -327.9141    65.0826    -5.04
0.000    -455.4736   -200.3545
>        _cons |   13099.53   1578.574     8.30   0.000     10005.58
16193.48
> -------------+------------------------------------------------------------
----
>     /sigma_u |   707.8464   527.0417     1.34   0.179    -325.1364
1740.829
>     /sigma_e |   2762.649   252.5473    10.94   0.000     2267.665
3257.632
> -------------+------------------------------------------------------------
----
>          rho |   .0616045   .0871224                      .0016116
.4454072
> --------------------------------------------------------------------------
----
>
>   Observation summary:        63     uncensored observations
>                               11  left-censored observations
>                                0 right-censored observations
>

> . 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
> --------------------------------------------------------------------------
-----
>
> . ****The marginal effects for the unconditional expected value of y
>
> .
>
>
> Hope this helps,
> Scott

the relation between  the mpg coefficients is: -252.7986/-327.9141=0.7709

the constant of the model that I'm interested in is:
13099.53*0.7709=10099

Thank you very much for your help and any hint you could give me about this.
Sincerely,
Marina Balboa

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