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:Other investigators using different experimental models, animals, and anesthetic agents experienced similar difficulty in stabilizing the circulatory volume when a crystalloid solution, saline, or Ringer solution was used to replace the blood lost (8, 12, 40, 41, 44, 55).
Category: