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:A 16S rDNA fragment was amplified by PCR (Hiraishi et al., 1994) using the universal primers 27F (59-AGAGTTTGATCATGGCTCGA-39; positions 8–27 in the Escherichia coli numbering system) and 1492R (59-GGCTACCTTGTTACGACTT-39; positions 1510–1492) (Weisburg et al., 1991).
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