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:LA Volume: The measurement of left atrial (LA) volume was obtained from the biplane method of disks (modified Simpson’s rule) using apical 4-chamber (A4C) and apical 2-chamber (A2C) views at ventricular end systole [7].
Category: