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:Thirty relevant studies concerning OPG polymorphisms and BMD were identified by MEDLINE and by hand searching, and 16 of these studies were selected for fulltext review based on their titles and abstracts [6–12, 17– 25].
Category: