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:Maximum metal levels chosen for the pilot experiment were based on published experiments in which mice or rats had been fed with Cu or Zn salts (Cunnane et al. 1986; Massie and Aiello 1979; Olafson 1983; Pocino et al. 1991, 1990; Sato et al. 1997; Zhang et al. 1994).
Category: