1. Biostatistics Unit, Medical Research Council, Durban, South Africa and Department of Statistics, University of Waikato, Hamilton, New Zealand
2. Biostatistics Unit, Medical Research Council, Durban, South Africa
3. Council for Scientific and Industrial Research, Johannesburg, South Africa
4. HIV Prevention Research Unit, Medical Research Council, Durban, South Africa and Brown University, School of Medicine, Providence RI, USA