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:In studies examining dose–response relationships between knee-straining work activities and degenerative knee disorders, retrospective exposure assessment has usually been based on self-reports (Felson et al. 1991; Vingard et al. 1991; Coggon et al. 2000; Sandmark et al. 2000; Seidler et al. 2008; Muraki et al. 2009; Klussmann et al. 2010).
Category: