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:While the prevalence of hyperechogenicity is only *9 % in healthy adults (Berg et al. 1999a), the statistical power was appropriate for detecting significant differences between groups (e.g. grip force group-by-trial interaction = 0.87).
Category: