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:Animals received either vascular occlusion (PM) for 15 min, or they underwent a standardized 2/3 liver resection (LR; left-lateral and median lobe) as described by Higgins and Andersson [17] or a combination of both procedures (PM + LR).
Category: