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:As discussed recently by Orsini et al. (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…
Category: