Department of Applied Statistics, PO Box 240, University of Reading, Reading RG6 6FN, UK. gmcguire@hgmp.mrc.ac.uk|Department of Applied Statistics, PO Box 240, University of Reading, Reading RG6 6FN, UK. gmcguire@hgmp.mrc.ac.uk|Department of Applied Statistics, PO Box 240, University of Reading, Reading RG6 6FN, UK. gmcguire@hgmp.mrc.ac.uk