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# Antwort: st: margins after logit model

 From Justina Fischer To statalist@hsphsun2.harvard.edu Subject Antwort: st: margins after logit model Date Thu, 30 Dec 2010 00:41:38 +0100

Hi

you have just three years.

A very easy way is to code two dummies for the years 2005 and 2007 each - the year not coded (2006) is then your base year in your regression.

Cheers

Justina

-----owner-statalist@hsphsun2.harvard.edu schrieb: -----

An: <statalist@hsphsun2.harvard.edu>
Von: Haiyong Xu <hxu@mednet.ucla.edu>
Gesendet von: owner-statalist@hsphsun2.harvard.edu
Datum: 30.12.2010 12:36AM
Thema: st: margins after logit model

Hi,
Wish you all have wonderful holidays!

My logit model and the output are as follows:

logit psydrgnobzn i.female age i.year, vce(cluster cluster_id)

Logistic regression                               Number of obs   =
40284
Wald chi2(4)    =
422.81
Prob > chi2     =
0.0000
Log pseudolikelihood = -26350.649                 Pseudo R2       =
0.0127

(Std. Err. adjusted for 13428 clusters in
cluster_id)
------------------------------------------------------------------------------
|               Robust
psydrgnobzn |      Coef.   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+----------------------------------------------------------------
1.female |   .5114119   .0357798    14.29   0.000     .4412848
.5815389
age |  -.0191765   .0023834    -8.05
0.000    -.0238479   -.0145051

|
year |
2006  |    .154101   .0147308    10.46   0.000     .1252292
.1829728
2007  |   .2495566   .0171274    14.57   0.000     .2159875
.2831257
|
_cons |   .5000495   .1866511     2.68   0.007       .13422
.865879
------------------------------------------------------------------------------

After the logit model, I run margins command to get the marginal effect.
. margins, dydx(year)

Average marginal effects                          Number of obs   =
40284
Model VCE    : Robust

_expression_   : Pr(psydrgnobzn), predict()
dy/dx w.r.t. : 2006.year 2007.year

------------------------------------------------------------------------------
|            Delta-method
|      dy/dx   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+----------------------------------------------------------------
year |
2006  |   .0351202   .0033485    10.49   0.000     .0285573
.0416832
2007  |   .0575292   .0039259    14.65   0.000     .0498345
.0652239
------------------------------------------------------------------------------
Note: dy/dx for factor levels is the discrete change from the base level.

The year variables has three values 2005, 2006 and 2007. Stata automatically
set 2005 as the omit group. I wonder if I can set 2006 as the reference
group so that I can get the margin effect between 2006 and 2007.

Thanks.
Haiyong

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