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:Although a broad array of research is needed to identify such a threshold dose for reinforcement (e.g., Goodwin et al. 2015; Grebenstein et al. 2015; Hatsukami et al. 2012; Hatsukami et al. 2013a; Hecht 2012), this dose likely is not lower than the threshold dose for discriminating nicotine’s…
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