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:Several edge-preserving filters (Hsieh 1998, Kachelriess et al 2001, Zhong et al 2004) have been proposed to reduce noise in CT images based on local characteristics of the projection data elements.
Category: