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Bahman Roudsari, MD MPH¹, Avery Nathens, MD PhD MPH², Thomas Koepsell, MD MPH¹, Charles Mock, MD PhD MPH¹, and Fredrick Rivara, MD MPH¹. (1) Epidemiology department, Harborview Injury Prevention and Research Center, University of Washington, Box: 359960 325 9th Ave, seattle, WA 98104, 206-744-9466, roudsari@u.washington.edu, (2) Department of Surgery and Harborview Injury Prevention and Research Center, University of Washington, Box: 359960 325 9th Ave,, seattle, WA 98104

In order to design multicenter studies having an estimate of the correlation of the observations within each center is necessary. A standard measure of the correlation between observations within each center is the Intraclass Correlation Coefficient (ICC).

We used the National Trauma Data Bank (NTDB). By 2004, 448 trauma centers (including 110 level I and 123 level II trauma centers) 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 centers was used to calculate the ICC of in-hospital trauma fatality and emergency department (ED) shock rate.

The ICCs of ED shock and in-hospital fatality rate were 0.010 (95% 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).

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 ICC's might have on sample size and power of their studies.

Learning Objectives:

• Multicenter trauma studies are becoming more and more common. However there are some methodological issues that should be taken into consideration in the design and and analysis of these studies.
• The outcome of the patients within each center might be correlated with each other. therefore the main assumption in standard (non-clustered) studies which is independency of observations, might be violated.
• This dependency of observations within each cluster require special sample size formula.
• Multicenter studies in general require larger sample size
• For sample size calculation, researchers should have some estimation about the Intracluster Correlation (ICC) of observation. Our study is the first study that has reported ICC for two of the most commonly used outcomes of interest.

Keywords: Injury, Methodology

Presenting author's disclosure statement:

Not Answered