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:Mean follow-up IOP measurement was obtained by averaging the IOP measured at 6-month intervals, and IOP fluctuation was determined using the SD of these values.(4) When patients underwent incisional glaucoma surgery or laser trabeculoplasty, the IOPs measured within 3 months posttreatment were not included in the calculation.
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