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:Two such factors predicting enhanced Delta REM (and thereby predicting substantial REM rebound) as suggested in several studies include: 1) a lower amount of REM sleep on diagnostic PSG (strong association) and 2) a higher severity of OSA as noted by the AHI during diagnostic PSG (weak association, if any).
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