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:While in various studies a high diagnostic accuracy for detecting SDB and a good correlation of AHI values has been shown for polygraphs compared with polysomnography [47-52], it is conceivable that the patients in this study population did not sleep well during the night before surgery and thus, the polygraphs might have underestimated the AHI scores in this setting.
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