title: Emergency department injury surveillance and aetiological research: bridging the gap with the two-stage case-control study design.
abstract: OBJECTIVE
  To provide an overview of the two-stage case-control study design and its potential application to ED injury surveillance data and to apply this approach to published ED data on the relation between brain injury and bicycle helmet use.
  
  
  METHODS
  Relevant background is presented on injury aetiology and case-control methodology with extension to the two-stage case-control design in the context of ED injury surveillance. The design is then applied to data from a published case-control study of the relation between brain injury and bicycle helmet use with motor vehicle involvement considered as a potential confounder. Taking into account the additional sampling at the second stage, the adjusted and corrected odds ratio and 95% confidence interval for the brain injury-helmet use relation is presented and compared with the estimate from the entire original dataset. Contexts where the two-stage case-control study design might be most appropriately applied to ED injury surveillance data are suggested.
  
  
  RESULTS
  The adjusted odds ratio for the relation between brain injury and bicycle helmet use based on all data (n = 2833) from the original study was 0.34 (95% CI 0.25 to 0.46) compared with an estimate from a two-stage case-control design of 0.35 (95% CI 0.25 to 0.48) using only a fraction of the original subjects (n = 480).
  
  
  CONCLUSIONS
  Application of the two-stage case-control study design to ED injury surveillance data has the potential to dramatically reduce study time and resource costs with acceptable losses in statistical efficiency.
authors.affiliation: Departments of Paediatrics and Community Health Sciences, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada. hagel@albertahealthservices.ca
authors.forename: Brent E
authors.initials: BE
authors.lastname: Hagel
mesh_terms: D001642:Bicycling / Q000293:injuries*; D001930:Brain Injuries / Q000209:etiology* / Q000517:prevention & control; D016022:Case-Control Studies; D004636:Emergency Service, Hospital / Q000191:economics / Q000706:statistics & numerical data*; D005260:Female; D006260:Head Protective Devices / Q000706:statistics & numerical data*; D006801:Humans; D008297:Male; D016017:Odds Ratio