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:Also, some approaches reduce the access time by caching [2, 3, 7, 18, 33], and nonuniform broadcasting [3, 18, 30, 31], and some reduce the tuning time by indexing [14, 19-21, 29, 33, 35], hashing [19] or using signature [24] techniques, where the tuning time is the amount of time spent by a client listening to the channel.
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