st: e(sample) after xtivreg2

[Chris Parker <cparker.phd2007@london.edu>]

Hello statalist,

I am using xtivreg2 from ssc to perform two-way error clustering.
However I would also like to look at the iid errors as well. I've
noticed that xtreg is faster that xtivreg2 so I wanted to calculate
the iid errors using xtreg and then use xtivreg2 only to calculate the
two-way clustered errors.

. xtreg highprice volume, fe

Fixed-effects (within) regression               Number of obs      =   1845327
Group variable: cropstat~tid                    Number of groups   =      8825

R-sq:  within  = 0.0005                         Obs per group: min =         1
       between = 0.0006                                        avg =     209.1
       overall = 0.0000                                        max =       873

                                                F(1,1836501)       =    852.88
corr(u_i, Xb)  = -0.0140                        Prob > F           =    0.0000

------------------------------------------------------------------------------
   highprice |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
      volume |  -.0037862   .0001296   -29.20   0.000    -.0040403   -.0035321
       _cons |   1595.633   .4527128  3524.60   0.000     1594.745     1596.52
-------------+----------------------------------------------------------------
     sigma_u |  3092.7207
     sigma_e |   602.9378
         rho |  .96338464   (fraction of variance due to u_i)
------------------------------------------------------------------------------
F test that all u_i=0:     F(8824, 1836501) =  1758.26       Prob > F = 0.0000
r; t=28.67 12:32:57

Which, on my four core license takes 28.67 seconds and uses 1,845,327
observations. Now when I run the same regression using xtivreg2 I am
issued a warning that singleton groups are detected and 172
observations will not be used:


. xtivreg2 highprice volume, fe
Warning - singleton groups detected.  172 observation(s) not used.

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =      8653                    Obs per group: min =         2
                                                               avg =     213.2
                                                               max =       873

OLS estimation
--------------

Estimates efficient for homoskedasticity only
Statistics consistent for homoskedasticity only

                                                      Number of obs =  1845155
                                                      F(  1,1836501) =   852.88
                                                      Prob > F      =   0.0000
Total (centered) SS     =  6.67941e+11                Centered R2   =   0.0005
Total (uncentered) SS   =  6.67941e+11                Uncentered R2 =   0.0005
Residual SS             =  6.67631e+11                Root MSE      =    602.9

------------------------------------------------------------------------------
   highprice |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
      volume |  -.0037862   .0001296   -29.20   0.000    -.0040403   -.0035321
------------------------------------------------------------------------------
Included instruments: volume
------------------------------------------------------------------------------
r; t=50.46 12:34:19

Just a note that this does in fact take longer, 50.46 seconds. Also,
the 172 observations are not included in the regression as noted by
number of obs. I thought it would be best to look at the xtreg
regression without the 172 observations. So I attempted to run the
xtreg regression with the sample from xtivreg2 only to find that all
of the observations are included in the sample from xtivreg2.

. gen mysample = e(sample)
r; t=0.20 12:35:04

. tab mysample

   mysample |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |  1,845,327      100.00      100.00
------------+-----------------------------------
      Total |  1,845,327      100.00
r; t=0.56 12:35:07

Can someone explain what is going on here? Is xtivreg2 actually using
the 172 observations or not?

My plan is to drop the singleton groups. However, I am running
regressions with different groups of independent variables with
missing observations. This could potentially create singleton groups
again. Is there a way to make xtreg ignore the singleton groups
automatically as well?

Additionally, does anyone know of a command that will let me calculate
the two-way clustered errors after xtreg? I don't really see the point
in running the same regression again just to cluster the errors
differently but short of coding the two-way clustered errors myself I
don't know how else to do this.

Chris


Chris Parker

________________________________

PhD Candidate | Management Science & Operations
London Business School | Regent's Park | London NW1 4SA | United Kingdom
Direct line +44 (0)20 7000 8816 | Email cparker.phd2007@london.edu
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