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:All the items in the questionnaire were adopted from the validated measurement tools which had been used in Skyrme and Amidon (1997a), Chase (1997), Chong et al. (2000) and KPMG Consulting (1998, 1999) studies.
Category: