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:Whereas Qazi et al. (2013) primarily manipulated channel selection and current levels within each temporal region in order to investigate the impact of noise-induced errors in sound coding strategies, the present study manipulated the gains that are applied in
a preceding noise reduction stage in…
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