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:To control for access to other home- and community-based services (HCBS), at the market level, defined by county boundaries, I controlled for home care capacity using the number of home health agencies per 1,000 people aged 65+ in the county (Hahn et al., 2011).
Category: