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 same 24 color pictures of Barbary macaques used in Pascalis et al. (2005) were used in this experiment to create 12 pairings (6 easy and 6 hard pairings).
Category: