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:In the case of a real option to invest, Nishimura and Ozaki (2007) show that ambiguity impacts the value of irreversible investments in a « drastically » different manner from that of “traditional uncertainty” (risk).
Category: