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:…back-propagation learning, Mixture-ofExperts (ME) model (Jacobs et al., 1991; Jordan & Jacobs, 1994; Jordan & Xu, 1995) and its alternative model (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…
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