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:The exercise of self-control was manipulated by assigning participants at random to one of two viewing conditions (similar to Muraven et al., 1998; Vohs & Heatherton, 2000; Vohs & Schmeichel, 2003).
Category: