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:…semi- or
nonparametric frontier models have been developed (e.g., Simar, 1992; Park and Simar, 1994; Fan et al., 1996; Kneip and Simar, 1996; Park et al., 1998, 2003, 2006; Post et al., 2002; Griffin and Steel, 2004; Henderson and Simar, 2005; Kuosmanen et al., 2007; and Kumbhakar et al., 2007).
Category: