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:Daptomycin is two- to fourfold more active in cationadjusted Mueller-Hinton broth containing 50 g of Ca(2) /ml than in medium containing 20 to 25 g of Ca(2) /ml, the concentration of Ca(2) presently recommended by the NCCLS (11).
Category: