1. Department of Mathematics and Statistics; University of Saskatchewan; Saskatoon SK S7N 5E6 Canada
2. CSIRO Computational Informatics; GPO Box 664 Canberra ACT 2601 Australia
3. Department of Statistics and Actuarial Science; University of Waterloo; Waterloo ON N2L 3G1 Canada
4. School of Public Health and Health Systems; University of Waterloo; Waterloo ON N2L 3G1 Canada