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:It is to be noted that the theories for SNRe and CNRe (Varghese and Ophir 1997R, 1998) provide upper bounds on the corresponding values and, therefore, the simulation and experimental results are expected to be lower than the theoretical values.
Category: