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:Recently, a few real option models characterized by the presence of Knightian uncertainty( or ambiguity) 8 have been proposed (Nishimura and Ozaki, 2007; Asano, 2005; Trojanowska and Kort, 2010; Miao and Wang, 2010).
Category: