Department of Epidemiology & Biostatistics, School of Rural Public Health, Texas A&M Health Science Center, College Station, TX 77843-1266, USA, School of Mathematics and Statistics, University of Sydney, NSW 2006 Australia, Department of Statistics, Texas A&M University, College Station, TX 77843-3143, USA and Department of Poultry Science, Intercollegiate Faculty of Nutrition, Texas A&M University, College Station, TX 77840, USA.