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:We refer those readers to the literature that tests and confirms the reliability of retrospective data in general (Berney and Blane 1997; Koenig et al. 2009), and retrospective smoking data in particular (Brigham et al. 2010; Kenkel et al. 2003).
Category: