1. School of Public Health; University of Alberta; Edmonton AB T6G1C9 Canada
2. Department of Mathematics and Statistics; Mississippi State University; Starkville Mississippi 39762 USA
3. Department of Statistics and Actuarial Science; Simon Fraser University; Burnaby B.C. V5A1S6 Canada
4. Fred Hutchinson Cancer Research Center, Seattle Children's Hospital; University of Washington; Seattle, Washington USA
5. Department of Epidemiology and Cancer Control; St. Jude Children's Research Hospital; 262 Danny Thomas Place, MS 735, Memphis TN 38105 USA