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:The validation study has shown that the root mean square differences for the hip and knee flexion/extension with three camera views were 2.6° and 7.5°, and multiple and perpendicular views were recommended to obtain more accurate results (Krosshaug and Bahr, 2005).
Category: