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:For example, the maximum total cardiac output at 23 ˚C in Reeves’s study was around 30–40 ml min21 kg21, a value considerably lower than that reported by Shelton and Burggren (1976).
Category: