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 recent years, a similar genotype shift has been observed in South Korea, northern Vietnam, China, Taiwan and Thailand (Chen et al., 2011; Nam et al., 1996; Nga et al., 2004; Nitatpattana et al., 2008; Pan et al., 2011; Wang et al., 2007; Yang et al., 2004; Yun et al., 2010; Zhang et al., 2009, 2011).
Category: