- Correspondence to: Dr M Hooiveld Department of Epidemiology and Biostatistics (133), Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB, Netherlands;
- Accepted 20 April 2006
- Published in print 1 August 2006.
- Published online 18 July 2006.
- Published first 6 June 2006.
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