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:Comparison of sperm retrieval rate according to testicular histology
Five of the included studies made a comparison of sperm retrieval according to testicular histology (Okada et al., 2002; Tsujimura et al., 2002; Ramasamy et al., 2005; Colpi et al., 2009; Ghalayini et al., 2011).
Category: