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:6), we had used different cohorts on each test day, but we subsequently verified that 2 or 3 tests can be performed in the same animals without affecting incubation (data not shown), as also found previously (Lu et al., 2004a; Li et al., 2008; Lu et al., 2009).
Category: