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:Then, the 29 pixels whose NDVI values were greater than 0 and less than 0.2 were seen as non-vegetation pixels (Tsai et al., 2007; Noujdina and Ustin, 2008).
Category: