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:6 %, which is in the middle range of findings of previous studies (3%-58%) (Massie and Holland, 1984; Ciaramella and Poli, 2001; Uchitomi et al., 2003; Katz et al., 2004; Ell et al., 2005; Boyd et al., 2012; Kim et al., 2012; Mitchell et al., 2012; Palmer et al., 2012; Pirl et al., 2012; Tada et al., 2012; Warmenhoven et al., 2012; Yu et al., 2012).
Category: