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:The rate of OJ− # production during reactions in vitro was measured as the superoxide dismutase (SOD)-sensitive rate of cytochrome c reduction (Imlay & Fridovich, 1991) in a Beckman DU 650 spectrophotometer.
Category: