MRC Biostatistics Unit, Robinson Way, Cambridge, CB2 0SR, UK. richard.newton@mrc-bsu.cam.ac.uk|MRC Biostatistics Unit, Robinson Way, Cambridge, CB2 0SR, UK. richard.newton@mrc-bsu.cam.ac.uk|MRC Biostatistics Unit, Robinson Way, Cambridge, CB2 0SR, UK. richard.newton@mrc-bsu.cam.ac.uk