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Many thanks to Martin Bland, Mario Cortina Borja,Mike Campbell, Kate Venables for helpful replies. Paula ******************************************************************************* Dr Paula Williamson Lecturer in Medical Statistics Department of Mathematical Sciences University of Liverpool Liverpool L69 3BX E-mail: [log in to unmask] Tel: 0151 794 4760 Fax: 0151 794 4754 ******************************************************************************* %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%