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:An alternative dose description approach, using pharmacokinetic analyses based on models described in the literature (Willems et al., 2001; Quick and Shuler, 1999; Sweeney et al., 1996; Frederick et al., 1998, 2001; NTP, 2000) was evaluated.
Category: