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:This constitutes an improvement over previous synoptic giant panda habitat evaluations in which the presence of understory bamboo was ignored (Liu et al., 2001; Loucks et al., 2003; Viña et al., 2007) due to the difficulty in isolating its signal from that of overstory canopy (Linderman et al.,…
Category: