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 superior bonding effectiveness shown in vitro [6, 7] and the resultant clinical performance [5, 8] has been attributed to optimal enamel inter-locking and dentin hybridization, as demonstrated in various ultramorphologic interface analyses [6, 7, 9].
Category: