Medical Research Council Biostatistics Unit, Cambridge, UK. wally.gilks@mrc-bsu.cam.ac.uk|Medical Research Council Biostatistics Unit, Cambridge, UK. wally.gilks@mrc-bsu.cam.ac.uk|Medical Research Council Biostatistics Unit, Cambridge, UK. wally.gilks@mrc-bsu.cam.ac.uk|Medical Research Council Biostatistics Unit, Cambridge, UK. wally.gilks@mrc-bsu.cam.ac.uk|Medical Research Council Biostatistics Unit, Cambridge, UK. wally.gilks@mrc-bsu.cam.ac.uk