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:For the first calculation, the denominator used was the cumulative ART exposure among adult patients at the referring ART clinics over that period, based on monthly reports of total patients in care in each referring clinic at the end of each month [15].
Category: