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:However, this was inconsistent with the investigations that showed a generally higher abundance of the tryptophan-like components in Lake Taihu than in its tributary samples (Yao et al. 2011; Zhou et al. 2015).
Category: