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 the assumption of tissue homogeneity in the probed volume may be adequate for small animal or infant human brain imaging [53], and may yield robust and reproducible measurements in the adult human brain [37], dependence of the optical coefficients to source detector separations have been reported in several studies [37,53,54,55,56,57], indicating a non-negligible depth dependence of the tissue composition.
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