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:Enzyme immunoassay (EIA) was performed for quantitative identification of 6-keto-PGF1 (the stable degradation product of PGI2) and TXB2 (the stable degradation product of TXA2) as described previously (20, 25).
Category: