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:An ideal day for scheduling OPU was defined by the administration of hCG on the day when at least three follicles reached the size of 17 mm, or with one day of delay (protocol charts 0 and +1), regardless of the type of protocol used, as supported by previous reports (3,9).
Category: