1. Ottawa Health Research Institute; Clinical Epidemiology Program; ASB Box 693 Rm 2-006, 1053 Carling avenue Ottawa Ontario Canada K1Y 4 E9
2. Ottawa Health Research Institute; Director, Clinical Epidemiology Programme; 1053 Carling Avenue Ottawa ON Canada K1Y 4E9
3. University of Aberdeen; Department of General Practice and Primary Care; Westburn Road Aberdeen UK AB25 2AY
4. University of Aberdeen; Department of Public Health, Medical School; Forresterhill Aberdeen UK AB25 2ZD
5. University of Aberdeen; Health Economics Research Unit; Aberdeen UK