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:Most of them, referred as background subtraction approaches, assume that the input video is captured by a stationary camera and model background colors at each pixel location using either generative [18, 15] or nonparametric [14, 5] methods.
Category: