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:To date, 28 cases of thyroglossal duct carcinoma with clinical information for patients younger than 20 years have been reported in the English literature [4,5,7,12-15]; the histologic diagnosis was PC in 27 cases [4,5,12-15] and mixed PC and SCC in 1 case [7].
Category: