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:The overall sound level of control and other auditory stimuli was roved from 50 to 80 dB SPL in steps of 2 dB.
Horizontal and vertical gaze positions were determined separately by a velocity criterion (Populin and Yin 1998; Tollin et al. 2005).
Category: