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:The 16S rRNA gene was sequenced for a random subset of 323 of the 459 isolates as described previously (Davelos et al., 2004c) (plot 1, n=146; plot 2N, n=51, plot 3, n=39; plot 4N, n= 27; plot 5, n= 42; plot 6N, n=18).
Category: