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:9 for a normalized kinetic energy of = 12 ( p= Y)v 2 0 = 0:2, corresponding to the Berkeley experiments wherein a steel ball impacts Ti–6Al–4V at v0 = 300 m=s (Peters et al., 2000, 2002; Boyce et al., 2001b).
Category: