1. Department of Mathematics and StatisticsMcGill UniversityMontreal Quebec Canada
2. Respiratory Epidemiology and Clinical Research UnitMcGill University Health CentreMontreal Quebec Canada
3. Department of EpidemiologyBloomberg School of Public HealthJohns Hopkins UniversityBaltimore MD USA
4. Department of MedicineMcGill UniversityMontreal Quebec Canada