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:…copy number (genome equivalents) per mitochondrial particle was determined from the total mtDAPI FI relative to DAPI FI from vaccinia virus using a method analogous to that described in (Miyamura et al. 1986) and used to determine ptDNA copy number (Oldenburg and Bendich 2004; Zheng et al. 2011).
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