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 have performed studies regarding lip hypoesthesia after SSRO using TSEP for long time (Nakagawa et al., 1997, 2001, 2003; Hashiba et al., 2007; Takazakura et al., 2007; Hashiba et al., 2008; Ueki et al., 2012, 2013a, 2013b).
Category: