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:On the contrary, when nonvertebrobasilar vertigo is considered, the combination of cervical MRI, TCD and BAEP has superiorities to rule out vertebrobasilar insufficiency for their negative correlations [14, 17].
Category: