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:The free cytosolic ADP concentration ([ADP]) was calculated according to the method previously described by Kemp and coworkers (33), assuming that PCr represents 85% of the total creatine content (Cr PCr) (7):
Category: