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:Although the lack of true branch lengths may increase Type I error of independent contrast correlations, equal branch lengths are associated with a lower Type I error than other topological approaches when information regarding true branch lengths does not exist (Ackerly 2000).
Category: