Methodological issues in the design of multi-center childhood injuries studies

Introduction: Design of a clustered childhood trauma study requires having an estimate of the variability in the outcome of interest between and within the clusters. “Intracluster correlation coefficient (ICC)” is a standard measure that captures these variabilities. However, ICC has not been reported for any childhood-related trauma outcome. Methods: We used the data from 246 level I and II trauma centers contributed data to the United States, National Trauma Data Bank to calculate the ICC of emergency department (ED) shock rate, early trauma death (i.e. death during the first 24 hours after admission) and in-hospital trauma death for children ≤ 15 years old. These outcomes are used for the evaluation of prehospital and hospital trauma care. Results: From 2000 to 2004, 13% of the 952,242 patients in NTDB were ≤ 15 years old . Approximately 17,000 of these children suffered severe injuries, of them 84% were hospitalized at a level I or II trauma center. In general, the ICC for ED shock rate (0.005, 95% CI: 0.000-0.010) was significantly less than the ICC for in-hospital trauma fatality rate (0.023, 95% CI: 0.013-0.033). A similar pattern was observed when we compared the ICC of ED shock rate with the ICC of in-hospital trauma fatality rate for different genders or types of injury. Conclusion: Clustered childhood trauma studies that aim at comparison of different aspects of prehospital and hospital trauma care should incorporate these ICCs for sample calculation.