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:…is because the mechanical and the sonographic parameters impose fundamental limitations and, hence, upper bounds on the quality of the strain estimates (Varghese and Ophir 1997), but the signal-processing parameters can be adjusted to obtain some, but not all, of these upper bounds simultaneously.
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