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:Considering the high level of contamination by chimeric sequences in the SILVA database [12], SILVA sequences were not counted towards cluster sizes, and clusters consisting only of SILVA sequences were excluded from further analysis (37,066 97% OTUs).
Category: