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:These anthropogenic sounds are generally more continuous in quality than sounds produced naturally in the environment (Wood & Yezerinac 2006; Slabbekoorn & Ripmeester 2008; Pijanowski et al. 2011).
Category: