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:Thus, the Patient Health Questionnaire-9 (PHQ-9) (Kroenke et al. 2001)‚Äîalready administered to students seeking services at the clinic‚Äîwas chosen as the screening instrument for the study.
Category: