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 overall sperm retrieval rate
All seven included studies compared the overall SRR (Schlegel, 1999; Amer et al., 2000; Okada et al., 2002; Tsujimura et al., 2002; Ramasamy et al., 2005; Colpi et al., 2009; Ghalayini et al., 2011).
Category: