st: estat phtest, detail --> different results using Stata10 and Stata11 if using robust option

[Valentin Gold <v.gold@web.de>]

I use "estat phtest, detail" after cox-regression to determine 
proportional hazards.
However, the results differ between Stata10 and Stata11 if I use the/a 
"robust" option in the Cox-regression.
Is there any reason for that?

Thanks in advance!

-----
webuse leukemia, clear
stset weeks, fail(relapse)
stcox treatment1 treatment2 wbc2 wbc3, scaledsch(sca*) schoenfeld(sch*) 
robust
estat phtest, detail

Stata 10 results:
. estat phtest, detail

      Test of proportional-hazards assumption

      Time:  Time
      ----------------------------------------------------------------
                  |       rho            chi2       df       Prob>chi2
      ------------+---------------------------------------------------
      treatment1  |      0.11966         0.64        1         0.4239
      treatment2  |     -0.30705         3.90        1         0.0484
      wbc2        |     -0.18233         0.99        1         0.3202
      wbc3        |     -0.01071         0.00        1         0.9562
      ------------+---------------------------------------------------
      global test |                      4.30        4         0.3670
      ----------------------------------------------------------------

note: robust variance-covariance matrix used.


Stata 11 results:
. estat phtest, detail

      Test of proportional-hazards assumption

      Time:  Time
      ----------------------------------------------------------------
                  |       rho            chi2       df       Prob>chi2
      ------------+---------------------------------------------------
      treatment1  |      0.06355         0.18        1         0.6749
      treatment2  |     -0.33518         4.55        1         0.0329
      wbc2        |     -0.10829         0.45        1         0.5034
      wbc3        |      0.00160         0.00        1         0.9912
      ------------+---------------------------------------------------
      global test |                      4.30        4         0.3670
      ----------------------------------------------------------------

note: robust variance-covariance matrix used.

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