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, analysis of data for 36, 424 older people in four developed countries found frailty index values to be closely comparable across countries, increasing with age at approximately 3% per year in community-dwellers and correlating highly with mortality [28].
Category: