[1] Consip SpA Natl Cent Purchasing Body, Div Concorrenza & Mercato, Rome, Italy
[2] Luiss Guido Carli, Dipartimento Econ & Finanza, Rome, Italy
[3] Univ Roma Tor Vergata, Dipartimento Econ & Finanza, Rome, Italy
[4] SOAS Univ London, Sch Finance & Management, London, England
[5] Univ Roma Tor Vergata, Dipartimento Econ & Finanza, Via Columbia 2, I-00133 Rome, Italy