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 ionomycin-induced increase in [Ca(21)]i without external Ca21, an index of the size of internal Ca21 stores (19, 20), was similar in patients with vasospastic angina and in control subjects (680 6 250 vs.
Category: