1. University Hospitals Coventry and Warwickshire NHS Trust, Clifford Bridge Road, Coventry, CV2 2DX, UK.
2. University of Birmingham, Public Health, Epidemiology and Biostatistics Unit, Birmingham B15 2TT, UK.
3. University of Warwick, Warwick Medical School, Clifford Bridge Road, Coventry CV2 2DX, UK.