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:The frequency of chronic of nausea reported in this study is consistent with results reported in previous studies [37, 39, 40]; however, compared to previous studies, which explored nausea related to chemotherapy [17, 34, 41], our study is one of the first to evaluate the specific problem of chronic nausea among outpatient advanced cancer patients receiving palliative care.
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