[1] Univ Reading, Sch Biol Sci, Reading RG6 5SA, Berks, England
[2] Helmut Schmidt Univ, Univ Fed Armed Forces Hamburg, Dept Econ & Org Sci, D-22039 Hamburg, Germany
[3] Univ Roma La Sapienza, Dipartimento Stat Probabilita & Stat Applicate, I-00185 Rome, Italy
[4] Inst Natl Polytech Toulouse, Grp Stat & Probabil, Inst Math Toulouse Stat & Probabil, ENSEEIHT,LEN7, F-31071 Toulouse 7, France
[5] Ecole Natl Stat & Anal Informat 1, Ctr Res Econ & Stat, F-35172 Bruz, France
[6] Stanford Univ, Dept Stat, Stanford, CA 94305 USA