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:Several studies retrospectively examine underage entry of females into the sex trade (Cobbina & Oselin, 2011; Goldenberg et al., 2014), but few neither include actively underage sex workers nor examine their social and structural HIV risks.
Category: