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:Recently, the echocardiographic proximal flow convergence method has been used to quantify the instantaneous mitral regurgitant flow and instantaneous effective regurgitant orifice area in patients with mitral regurgitation (14,15).
Category: