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:Risk factors for perforation include older age, difficulty and length of the procedure, length of sphincterotomy, periampullary diverticulum of the duodenum, abnormal anatomy (Billroth II gastrectomy), dilated bile duct, biliary duct stricture, sphincter of Oddi dysfunction, and papillary stenosis [16–18].
Category: