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:Predisposing factors associated with NE have been used in challenges to increase the potential for disease replication with varying results (Olkowski et al., 2006; Chalmers et al., 2007; Gholamiandehkordi et al., 2007).
Category: