About the Authors

Gordon C. S Smith

To whom correspondence should be addressed. E-mail: gcss2@cam.ac.uk

Affiliation Department of Obstetrics and Gynaecology, Cambridge University, Cambridge, United Kingdom

Ian R White

Affiliation Medical Research Council Biostatistics Unit, Institute of Public Health, Cambridge, United Kingdom

Jill P Pell

Affiliation Department of Public Health, Greater Glasgow NHS Board, Glasgow, United Kingdom

Richard Dobbie

Affiliation Information and Statistics Division, Common Services Agency, Edinburgh, United Kingdom

Competing Interests

The authors have declared that no competing interests exist.

Author Contributions

GCSS designed the study. RD collected the data. IRW developed methods. GCSS and JPP analyzed the data. GCSS, IRW, JPP, and RD contributed to writing the paper.