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 studies have shown that hypertension is a good predictor of RAS in patients with CAD or undergoing coronary angiography.(19,20) However, in some studies, hypertension failed to predict the presence of RAS.
Category: