title: Analysis of clustered data in multicentre trauma studies.
abstract: BACKGROUND
  In order to design multicentre studies an estimate of the correlation of the observations within each centre is necessary. A standard measure of the correlation between observations within each centre is the Intraclass Correlation Coefficient (ICC).
  
  
  METHODS
  We used the National Trauma Data Bank (NTDB). By 2004, 448 trauma centres (including 110 level I and 123 level II trauma centres) from 43 states and US territories contributed over 1.2 million records to the NTDB. Data of patients directly transported from the scene of injury to level I or II trauma centres were used to calculate the ICC of in-hospital trauma fatality and emergency department (ED) shock rate.
  
  
  RESULTS
  The ICCs of ED shock and in-hospital fatality rate were 0.010 (95% confidence interval (CI): 0.003-0.018) and 0.039 (95% CI: 0.028-0.050), respectively. The ICC of shock in the ED was the highest for penetrating injuries (0.017, 95% CI: 0.003-0.032) and the lowest for women (0.008, 95% CI: 0.002-0.013) although the observed difference between men and women was not statistically significant. The ICC of trauma fatality was the highest for penetrating injuries (0.073, 95% CI: 0.047-0.098), and the lowest for blunt injuries (0.029, 95% CI: 0.020-0.037).
  
  
  CONCLUSIONS
  Although the calculated ICCs might seem so small as to be ignored, the required sample size in studies with exclusively exposed or non-exposed clusters depends on the ICC and the average number of subjects within clusters. Therefore, investigators should be aware of the influence that these ICCs might have on sample size and power of their studies.
authors.affiliation: Harborview Injury Prevention and Research Center, University of Washington, Seattle, WA, USA. roudsari@u.washington.edu
authors.forename: Bahman
authors.initials: B
authors.lastname: Roudsari
authors.forename: Avery
authors.initials: A
authors.lastname: Nathens
authors.forename: Thomas
authors.initials: T
authors.lastname: Koepsell
authors.forename: Charles
authors.initials: C
authors.lastname: Mock
authors.forename: Frederick
authors.initials: F
authors.lastname: Rivara
mesh_terms: D000328:Adult; D000368:Aged; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012107:Research Design; D012774:Shock, Traumatic / Q000453:epidemiology / Q000209:etiology; D014193:Trauma Centers; D014481:United States / Q000453:epidemiology; D014947:Wounds and Injuries / Q000401:mortality / Q000628:therapy*