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:Such an FBAT statistic can be the standard TDT, an FBAT for quantitative/qualitative traits, FBAT-GEE for multivariate traits, etc [4,6,18,20,21].
Category: