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:Our definition of bipolar NOS has been validated and used in previous studies (Akiskal and Benazzi, 2005; Benazzi and Akiskal, 2006; Kim et al., 2008; Lee et al., 2009).
Category: