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:Annual public expenditure on home-care services tripled from €102.3 million in 2001 (Timonen et al. 2006a) to €331 million in 2008 (PA Consulting 2009b).
Category: