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:In selecting REM epochs, we took into account the presence of a clearly defined alpha burst in any EEG derivation, the total absence of spindle activity in frontoparietal regions (Jobert et al. 1992), the appearance of rapid eye movements one minute before, and on continuing in REM stage, one minute after the selected epoch.
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