1. Department of Epidemiology and Biostatistics, Institute of Public Health, Kilimanjaro Christian Medical University College (KCMUCo), Moshi P.O. Box 2240, Tanzania
2. Department of Population Health, London School of Hygiene and Tropical Medicine, London UK and National Institute for Medical Research, Mwanza P.O. Box 1708, Tanzania
3. School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Pietermaritzburg 3209, South Africa
4. Department of Mathematical Sciences, Strathmore University, Kitale P.O. Box 850-30200, Kenya