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:Patients with PD and elderly fallers walk more slowly when asked to perform a cognitive task simultaneously, with a reduced swing time and with an increased stride variability (Bond and Morris 2000; O’Shea et al. 2002; Springer et al. 2006; Yogev et al. 2005).
Category: