[Richard Williams]

Couldn't the existence of ties be checked?

Excellent point. You don't just need to guess or hope and pray! You can also use the duplicates command, e.g. using Robert's data,

. duplicates report alpha, force

Duplicates in terms of alpha

--------------------------------------
   copies | observations       surplus
----------+---------------------------
        1 |         1000             0
--------------------------------------

. expand 2 if _n < = 10
(10 observations created)

. duplicates report alpha, force

Duplicates in terms of alpha

--------------------------------------
   copies | observations       surplus
----------+---------------------------
        1 |          990             0
        2 |           20            10
--------------------------------------

Lander probably won't have any duplicates on alpha, but if he does he is only worried about duplicates within a panel. So he could do

duplicates report panelid alpha, force

[Nick Cox]

There were some good answers here; nevertheless everyone missed http://www.stata.com/support/faqs/st...ons/index.html