1. Department of Paediatrics: Child and Youth Health, The University of Auckland, Auckland 1142, New Zealand.
2. Wellington Asthma Research Group, Wellington School of Medicine, University of Otago, Wellington 6242, New Zealand.
3. Canterbury Health Laboratories, Christchurch 8140, New Zealand.
4. Bay of Plenty District Health Board, Whakatane 3158, New Zealand.
5. Department of Paediatrics, Christchurch School of Medicine, University of Otago, Christchurch 8140, New Zealand.
6. Department of Medical Statistics, London School of Hygiene and Tropical Medicine, London WC1E 7HT, United Kingdom.
7. School of Population Health, The University of Auckland, Auckland 1142, New Zealand.