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:Achenbach (1991) has shown that the YSR and its subscales are reliable and valid measures of problematic behaviors among clinic-referred and nonreferred youths, and further research documents that the measure is reliable and valid for youths as young as 7 years old (Ebesutani et al. 2011; Yeh and Weisz 2001).
Category: