^aDepartment of Epidemiology and Public Health, University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK

^bPain Relief Unit, Churchill Hospital, Headington, Oxford OX3 7LJ, UK

Received 19 January 1998; accepted 28 January 1998

^* Corresponding author. Tel.: +44 382 60111, ext. 3155; fax: +44 382 644197.