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:All 7 transabdominal ultrasound studies [24,27,57–61] 11 of 22 [28,30,41–44,48,50–52,55] EUS studies used contrast enhancement.
Category: