MRC Integrative Epidemiology Unit (IEU), University of Bristol, Bristol, BS8 2BN, UK.|Intelligent Systems Laboratory, University of Bristol, Bristol, BS8 1UB, UK.|Intelligent Systems Laboratory, University of Bristol, Bristol, BS8 1UB, UK.|Intelligent Systems Laboratory, University of Bristol, Bristol, BS8 1UB, UK.|MRC Integrative Epidemiology Unit (IEU), University of Bristol, Bristol, BS8 2BN, UK. tom.gaunt@bristol.ac.uk.