st: Oaxaca Decomposition with Deviation Transform Coding

[Ian Li <ianz.magic@hotmail.com>]

Dear Statalist,

I would like to perform a Blinder-Oaxaca decomposition that accounts for the choice of the reference category for categorical variables (see Jones and Kelley (1984) "Decomposing Differences Between Groups: A Cautionary Note on Measuring Discrimination", Sociological Methods and Research). I use the -oaxaca- command written by Ben Jann, and has also read the Stata Journal article written by him on the use of this command (Jann (2008) "The Blinder-Oaxaca Decomposition for Linear Regression Models", The Stata Journal vol. 8, no. 4, pp. 453-479).

In estimating the model using the 'categorical' command to perform deviation transform coding, however, I get the exact same result to that when the 'categorical' option is not specified. This makes me suspicious of the output result. However, I can't see any mistakes with my specification of the command. I have copied the output below, and would be grateful to anyone who can point out where I have gone wrong. 

Regards
Ian

 do "C:\Users\Ian\AppData\Local\Temp\STD00000000.tmp"

. oaxaca lnhrsal natphysci infotech engineer archi agrienv nursing medicine educ soccult creatoth go8 tech iru if empnat1 == 
> 1, by(female) weight(0.5) 

Blinder-Oaxaca decomposition                      Number of obs   =     569325

           1: female = 0
           2: female = 1

-------------------------------------------------------------------------------
      lnhrsal |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
--------------+----------------------------------------------------------------
Differential  |
 Prediction_1 |   3.065791   .0013305  2304.22   0.000     3.063184    3.068399
 Prediction_2 |   2.970103   .0010395  2857.33   0.000     2.968066    2.972141
   Difference |    .095688   .0016884    56.67   0.000     .0923787    .0989972
--------------+----------------------------------------------------------------
Decomposition |
    Explained |   .0041661   .0008113     5.13   0.000     .0025759    .0057563
  Unexplained |   .0915218   .0018572    49.28   0.000     .0878817     .095162
-------------------------------------------------------------------------------

. 
end of do-file

. do "C:\Users\Ian\AppData\Local\Temp\STD00000000.tmp"

. oaxaca lnhrsal natphysci infotech engineer archi agrienv nursing medicine educ soccult creatoth go8 tech iru if empnat1 == 
> 1, by(female) weight(0.5) categorical (natphysci infotech engineer archi agrienv nursing medicine educ soccult creatoth mgt
> comm, go8 tech iru otheruni)

Blinder-Oaxaca decomposition                      Number of obs   =     569325

           1: female = 0
           2: female = 1

-------------------------------------------------------------------------------
      lnhrsal |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
--------------+----------------------------------------------------------------
Differential  |
 Prediction_1 |   3.065791   .0013305  2304.22   0.000     3.063184    3.068399
 Prediction_2 |   2.970103   .0010395  2857.33   0.000     2.968066    2.972141
   Difference |    .095688   .0016884    56.67   0.000     .0923787    .0989972
--------------+----------------------------------------------------------------
Decomposition |
    Explained |   .0041661   .0008113     5.13   0.000     .0025759    .0057563
  Unexplained |   .0915218   .0018572    49.28   0.000     .0878817     .095162
-------------------------------------------------------------------------------

. 
end of do-file


 		 	   		   		 	   		  
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