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 measurements were taken on a Morgan Benchmark lung function testing apparatus (PK Morgan ltd., Kent, England), and were done by the same technician and according to the standardized procedures of the European Respiratory Society (Quanjer et al. 1993; Cotes et al. 1993).
Category: