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:The KOOS score has been used in numerous clinical studies on knee injuries,(9,19,52,55,57) and the literature supports both acceptable test-retest reliability and validity.(9,19,56,57) The KOOS consists of 5 subscales: (1) pain, (2) other symptoms, (3) function in activities of daily living (ADLs), (4) function in sports and recreation (sports), and (5) knee-related quality of life (QOL).
Category: