Random tiebreaking with –sort–

With –sort– I'm aware that unless the –stable– option is specified that ties among the sort varlist are broken randomly. That said, can anyone inform me why I get different sort orders when I run the following code multiple times.

Code:

cap preserve
cap drop _all

set seed 2345
set obs 6
gen subject=_n
gen u=runiform()
gen var=ceil(_n/3)
list
sort u
sort var
list
sort u
sort var
list
sort u
sort var
list

cap restore

First execution:

Code:

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       1   .2763898     1 |
  2. |       2   .1392859     1 |
  3. |       3   .1685803     1 |
  4. |       4   .2651878     2 |
  5. |       5   .5484495     2 |
     |--------------------------|
  6. |       6   .3501784     2 |
     +--------------------------+

. sort u

. sort var

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       1   .2763898     1 |
  2. |       2   .1392859     1 |
  3. |       3   .1685803     1 |
  4. |       6   .3501784     2 |
  5. |       4   .2651878     2 |
     |--------------------------|
  6. |       5   .5484495     2 |
     +--------------------------+

. sort u

. sort var

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       3   .1685803     1 |
  2. |       2   .1392859     1 |
  3. |       1   .2763898     1 |
  4. |       5   .5484495     2 |
  5. |       6   .3501784     2 |
     |--------------------------|
  6. |       4   .2651878     2 |
     +--------------------------+

. sort u

. sort var

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       2   .1392859     1 |
  2. |       1   .2763898     1 |
  3. |       3   .1685803     1 |
  4. |       5   .5484495     2 |
  5. |       6   .3501784     2 |
     |--------------------------|
  6. |       4   .2651878     2 |
     +--------------------------+

Second execution:

Code:

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       1   .2763898     1 |
  2. |       2   .1392859     1 |
  3. |       3   .1685803     1 |
  4. |       4   .2651878     2 |
  5. |       5   .5484495     2 |
     |--------------------------|
  6. |       6   .3501784     2 |
     +--------------------------+

. sort u

. sort var

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       2   .1392859     1 |
  2. |       1   .2763898     1 |
  3. |       3   .1685803     1 |
  4. |       6   .3501784     2 |
  5. |       5   .5484495     2 |
     |--------------------------|
  6. |       4   .2651878     2 |
     +--------------------------+

. sort u

. sort var

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       3   .1685803     1 |
  2. |       2   .1392859     1 |
  3. |       1   .2763898     1 |
  4. |       5   .5484495     2 |
  5. |       4   .2651878     2 |
     |--------------------------|
  6. |       6   .3501784     2 |
     +--------------------------+

. sort u

. sort var

. list

     +--------------------------+
     | subject          u   var |
     |--------------------------|
  1. |       2   .1392859     1 |
  2. |       1   .2763898     1 |
  3. |       3   .1685803     1 |
  4. |       5   .5484495     2 |
  5. |       6   .3501784     2 |
     |--------------------------|
  6. |       4   .2651878     2 |
     +--------------------------+

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