1. Department of Statistics and Actuarial Science, The University of Hong Kong , Pokfulam Road , Hong Kong
2. Department of Biostatistics, University of Michigan , 1415 Washington Heights, Ann Arbor , Michigan 48109, USA
3. Department of Epidemiology, University of North Carolina at Chapel Hill , 137 East Franklin Street , Chapel Hill, North Carolina 27599, USA
4. Department of Biostatistics, University of North Carolina at Chapel Hill , 3101E McGavran-Greenberg Hall , Chapel Hill, North Carolina 27599, USA