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:Complications were scored according to the Modified Accordion Severity Grading System, and quantified using previously validated severity utility weights.(12,13) The postoperative morbidity index was calculated by dividing the total burden of highest-grade complications by the total number of patients in any given cohort studied, including those without any complications (which were included in the calculations as zeros).
Category: