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:Previous reports have shown that the indiscriminate use of axillary ultrasound alone or in combination with fine needle aspiration or core biopsies in all breast cancer patients is not practical or cost-effective [12, 13, 16, 26– 31]; however, as stressed in this review, when this approach is targeted to a breast cancer population at higher risk for axillary metastases, the approach is not only clinically justified but is also cost-effective [18].
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