1. Department of Epidemiology and Biostatistics, Arnold School of Public Health, University of South Carolina , Columbia, SC 29208, USA
2. Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina , Columbia, SC 29208, USA
3. Department of Quantitative Sciences, Baylor College of Medicine , Houston, TX 77030, USA
4. Department of Biostatistics, University of Florida , Gainesville, FL 32603, USA