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:Some of the reasons for the discrepancies include lateral and elevational motion (Kallel et al. 1997) and frequencydependent attenuation (Varghese and Ophir 1997c), low SNRs, nonhomogeneity of the phantoms, and nonuniform compression.
Category: