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:Two studies comprehensively analyzed all available full-length HBV genomes in GenBank/EMBL/DDBJ and reported the frequent occurrence of recombination event between two or more HBV strains [14,15].
Category: