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 fact, Underwood (1957) made this very point when he observed that the sleeping subjects in Jenkins and Dallenbach’s (1924) classic sleep study did not avoid learning material that was similar to the study list, so avoidance of retroactive interference could not explain why sleep facilitates…
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