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:Previous research find that the VOT value of aspirated decreases and overlaps with that of lenis for younger speakers, but not for older speakers (Jin 2008, Kang 2014, Kang and Guion 2008, Silva 2006, Wright 2007).
Category: