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 TLFBmethod (Sobell and Sobell 1992) was used to obtain self-reported amount and frequency of cannabis use in the past 30 days, and diagnosis of current cannabis use disorders was conducted using the DSM checklist (Hudziak et al. 1993).
Category: