1 Department of Biostatistics & Epidemiology, School of Public Health & Health Sciences, University of Massachusetts, 401 Arnold House, 715 North Pleasant Street, Amherst, MA, 01003-9304, USA. LCT@schoolph.umass.edu.
2 Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina - Chapel Hill, Chapel Hill, NC, USA.