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:Postoperative intubation time (hrs) median (IQR [range]) 13 (10-18 [4-91]) 11 (9-14 [5–29]) 18 (14-23[4-91]) <0.
Category: