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:Statistical analysis of EEG SSDC as applied to the frontal–central and parietooccipital cortical areas also confirmed the previously demonstrated [2] zonal speci ficity of SSDC in the MM–REO and RM–REO con trasts.
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