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:Previous studies indicate that jellyfish planulae have different substrate choices and settlement preferences between natural and artificial materials (Holst & Jarms, 2007; Hoover & Purcell, 2009; Duarte et al., 2012).
Category: