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:Competition of BaP and BeP with [3H]2,3,7,8tetrachlorodibenzo-p-dioxin ([3H]TCDD) for binding to a trout AHR2αwasmeasured by velocity sedimentation in sucrose gradients in a vertical tube rotor (Tsui and Okey, 1981) as described earlier (Billiard et al., 2002; Karchner et al., 1999).
Category: