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:Another approach is to fit a distribution of selective effects to the SFS (Eyre-Walker et al., 2006; Keightley and Eyre-Walker, 2007; Boyko et al., 2008; Schneider et al., 2011).
Category: