¹Department of Information Systems, London School of Economics and Political Science, London, U.K.

Correspondence: Diego D. Navarra, Department of Information Systems, London School of Economics and Political Science, London WC2 2AE, U.K. E-mail: d.d.navarra@lse.ac.uk

Received 8 August 2005; Revised 26 September 2005; Accepted 28 September 2005.