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:Therefore, we tested three morphine-like drugs that represent a range of efficacy: morphine (intermediate efficacy), buprenorphine (relatively low efficacy), and etorphine (relatively high efficacy) (Emmerson et al. 1996; Selley et al. 1998) on the hot-plate and tail-flick tests under conditions previously described (Paronis and Holtzman 1991; Kalinichev et al.
Category: