On Fri, 26 Aug 2011, Allin Cottrell wrote:
 I'll do some more checking tomorrow to confirm that the fix is
 right, but thanks for spotting this! 
Confirmed, the fix looks ok to me.
I'm attaching a zip file containing Skipper's dataset and a small script 
performing the relevant computations.
Riccardo (Jack) Lucchetti
Dipartimento di Economia
Università Politecnica delle Marche
r.lucchetti(a)univpm.it
http://www.econ.univpm.it/lucchetti