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:We tested model performance using the area under the receiver operating curve (AUC) statistic, which gives a measure of ability to discriminate between observed presences and absences, and has been widely applied (Acevedo et al. 2010; Anderson and Raza 2010; Phillips et al. 2006; but see Lobo et al. 2008).
Category: