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:For the patients in the register cohort, except those diagnosed in Stockholm prior to 1999, stage, treatment, and relapse information were gathered from two clinical data sources: the National Database of HL [18, 19] (1992–2009, covering all regions except Stockholm) and the Swedish Lymphoma Register in the Information Network for Cancer Care (2000 and onward) [20].
Category: