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RE: st: RE: mlogit question.


From   "Scott Merryman" <smerryman@kc.rr.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: mlogit question.
Date   Fri, 15 Apr 2005 13:58:22 -0500

Richard,

Does the base category matter?  Since the coefficients are difficult to
interpret (i.e. the direction of the effect of var_x on outcome 1 is not
determined solely estimated coefficient of var_x for outcome 1 (Wooldridge,
2002 p. 497-498)) wouldn't you usually compute marginal effects, which do
not depend on the base category?

For example, the sign on _mpg_ changes sign when the base category changes
from 3 to 2, but the marginal effect is the same.


. mlogit rep price mpg, base(3) nolog

----------------------------------------------------------------------------
       rep78 |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+--------------------------------------------------------------
1            |
       price |  -.0006468    .000849    -0.76   0.446    -.0023108
.0010173
         mpg |  -.0197515   .1959901    -0.10   0.920    -.4038851
.364382
       _cons |   1.013502   6.695462     0.15   0.880    -12.10936
14.13637
-------------+--------------------------------------------------------------
<snip>

 (Outcome rep78==3 is the comparison group)


. mfx , predict(outcome(1)) var(mpg)

Marginal effects after mlogit
      y  = Pr(rep78==1) (predict, outcome(1))
         =  .01570292
----------------------------------------------------------------------------
variable |      dy/dx    Std. Err.     z    P>|z|  [    95% C.I.   ]      X
---------+------------------------------------------------------------------
     mpg |  -.0012526      .00287   -0.44   0.662  -.006868  .004363
21.2899
----------------------------------------------------------------------------

. mlogit rep price mpg, base(2) nolog

----------------------------------------------------------------------------
       rep78 |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+-------------------------------------------------------------
1            |
       price |  -.0005451   .0008602    -0.63   0.526    -.0022309
.0011408
         mpg |   .0330037   .2163959     0.15   0.879    -.3911245
.4571319
       _cons |   .6890743   7.076853     0.10   0.922     -13.1813
14.55945
-------------+--------------------------------------------------------------
<snip>

 (Outcome rep78==2 is the comparison group)

. mfx , predict(outcome(1)) var(mpg)

Marginal effects after mlogit
      y  = Pr(rep78==1) (predict, outcome(1))
         =  .01570292
----------------------------------------------------------------------------
variable |      dy/dx    Std. Err.     z    P>|z|  [    95% C.I.   ]      X
---------+------------------------------------------------------------------
     mpg |  -.0012526      .00287   -0.44   0.662  -.006868  .004363
21.2899
----------------------------------------------------------------------------


Scott


Wooldridge, Jeffery.  "Econometric Analysis of Cross Section and Panel
Data." Cambridge, MA: MIT Press, 2002.



> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
> statalist@hsphsun2.harvard.edu] On Behalf Of Richard. Williams
> Sent: Friday, April 15, 2005 11:32 AM
> To: statalist@hsphsun2.harvard.edu; statalist@hsphsun2.harvard.edu
> Subject: Re: st: RE: mlogit question.
> 
> At 09:59 AM 4/15/2005, Anna-Cristina.DADDIO-DEVAUX@oecd.org wrote:
> >yes,
> >you should use the option basecategory(#) after the command
> >mlogit depvar [indepvars] [weight] [if exp] [in range] ,  basecategory(#)
> >anna
> 
> I've always found mlogit's practice of using the most frequent category as
> the base category a little annoying.  It is easy enough to override, but
> it
> would be nice to be able to say something like basecategory(last) for when
> you want to use the last category as the base but don't happen to remember
> what its value is.  It would also be nice if you could permanently set it
> to go with the first or the last value, as I'm always forgetting to
> override the default behavior.
> 
> 
> -------------------------------------------
> Richard Williams, Associate Professor
> OFFICE: (574)631-6668, (574)631-6463
> FAX:    (574)288-4373
> HOME:   (574)289-5227
> EMAIL:  Richard.A.Williams.5@ND.Edu
> WWW (personal):    http://www.nd.edu/~rwilliam
> WWW (department):    http://www.nd.edu/~soc
> 
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