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:DNA was extracted from 0.25 g of each pool using the repeated bead beating plus column method of Yu and Morrison [24] and then used for estimation of numbers of target bacteria using quantitative real-time PCR (qRT-PCR).
Category: