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 this article, we report absolute measurements of the optical coefficients of two-layered media that expand on our previous work on multi-distance absolute measurements on tissue-like phantoms [66], animal brain [35,51], and human brain [37].
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