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:…locations (Benke et al. 1999; Billinton et al. 1999; Bischoff et al. 1999;
Chapter 1 33 _________________________________________________________________________________
Kaupmann et al. 1998b; Liang et al. 2000; Poorkhalkali et al. 2000; Princivalle et al. 2000;
Towers et al. 2000).
Category: