1. Biostatistics and Research Decision Sciences, Merck Research Laboratories, Merck & Co., Inc., Rahway, NJ 07065, USA
2. Department of Biostatistics, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205, USA
3. Biostatistics and Research Decision Sciences, Merck Research Laboratories, Merck & Co., Inc., North Wales, PA 19454, USA