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:Several authors (Anderson et al., 2009; Malmierca et al., 2009; Ulanovsky et al., 2004, 2003) use the normalized frequency difference:
f norm, f2 − f1√
f1 f2 (A.1)
where f1 and f2 have Hertz units, while others use the octave frequency separation (e.g., Von der Behrens et al., 2009).
Category: