1. Department of Mathematics and Statistics; Queen's University; Kingston Canada ON K7L 3N6
2. Department of Mathematics and Statistics; Acadia University; Wolfville Canada NS B4P 2R6
3. Westat and the Joint Program in Survey Methodology; 1600 Research Boulevard; Rockville MD 20850-3129 USA
4. Department of Statistics and Actuarial Science; Simon Fraser University; Burnaby, BC Canada V5A 1S6