title: Modeling smoking in systemic sclerosis: a comparison of different statistical approaches.
abstract: OBJECTIVE
  To determine the effect of different methods of modeling smoking on vascular outcomes in rheumatic diseases.
  
  
  METHODS
  Data from the Canadian Scleroderma Research Group Registry were used. Patients self-reported their smoking history. Vascular outcomes were severity of Raynaud's phenomenon, presence of finger ulcers, and severity of finger ulcers. Several models were developed to capture the experience of smoking: 1) ever compared to never smoking; 2) current and past smoking compared to never smoking; 3) never, past, and current smoking compared using polynomial contrasts; 4) smoking intensity, duration, and time since cessation assessed separately; and 5) smoking modeled using the Comprehensive Smoking Index (CSI), which integrates intensity, duration, and time since cessation into a single covariate.
  
  
  RESULTS
  This study included 606 patients, of which 16% were current, 42% were past, and 42% were never smokers. Current and past smokers smoked a mean±SD of 25±17 and 17±18 pack-years, respectively. Smoking duration was shorter in past compared to current smokers (18.3 versus 31.7 years). Past smokers reported having stopped smoking approximately mean±SD 16±12 years prior, although this ranged from 1 to 50 years. Smoking had no effect on vascular outcomes in the simplest model comparing ever to never smokers. Models that isolated past smokers revealed the presence of a healthy smoker bias in that group. The model using the CSI demonstrated a strong negative effect of smoking on vascular outcomes.
  
  
  CONCLUSIONS
  Proper modeling of the effect of smoking is essential in studies of vascular outcomes of rheumatic diseases.
authors: Jewish General Hospital and McGill University, Montreal, Quebec, Canada. marie.hudson@mcgill.ca
authors: Marie
authors: M
authors: Hudson
authors: Ernest
authors: E
authors: Lo
authors: Murray
authors: M
authors: Baron
authors: Russell
authors: R
authors: Steele
mesh_terms: D000328:Adult; D000368:Aged; D001699:Biometry / Q000379:methods; D002170:Canada / Q000453:epidemiology; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D012042:Registries / Q000706:statistics & numerical data*; D012595:Scleroderma, Systemic / Q000453:epidemiology* / Q000209:etiology / Q000473:pathology; D012720:Severity of Illness Index; D012907:Smoking / Q000009:adverse effects / Q000453:epidemiology* / Q000473:pathology