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:…(Xu, 1998a; Xu et al., 1995), as well as normalized RBF nets (Moody & Darken, 1989; Nowlan, 1990; Xu et al., 1994) and its extensions (Xu, 1998a), but also new adaptive algorithms for learning and new criteria for deciding the number of hidden units, experts, and basis functions (Xu, 1998a, 2001b).
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