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:Prior work on theSNRe (Varghese and Ophir 1997a) and CNRe (Varghese and Ophir 1998) has established theoretical upper bounds on the values ofSNRe and CNRe.
Category: