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:…(2005), several differences between CPP protocols such as the number of compartments in the CPP apparatus as well as the usage of biased (as in Cunningham et al. 1992; Semenova et al. 1995) vs unbiased procedure (as in Orsini et al. 2005) might account for some of the discrepancies in the results.
Category: