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:…one another in many acoustic features, such as Voice Onset Time (VOT), fundamental frequency, and the amplitude difference between the first harmonic and the second one (Cho et al. 2002, Han and Weitzman 1970, Kang 2014, Lisker and Abramson 1964, Oh 2011, Silva 2006, Wright 2007, among others).
Category: