Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:As in previous studies, injury severity was stratified using the discharge diagnosis ICD-9ebased ICISS scoring system, which has been shown to outperform other scoring systems in predicting injury mortality and is particularly suited for the study of administrative databases.(8,9,12,13) The definition of severely injured patients was chosen to match that in several preceding studies that focused on the survival advantage of severely injured patients treated at DTCs in Florida.
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