1. Department of Biostatistics, Faculty of Health and Life Sciences; University of Liverpool; Shelley's Cottage, Brownlow Street; Liverpool; L69 3GS; U.K.
2. Prince Leopold Institute of Tropical Medicine; Nationalestraat 155; B-2000 Antwerp; Belgium
3. Liverpool School of Tropical Medicine; Pembroke Place; Liverpool; L3 5QA; U.K.