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:…effects of reinforcers and punishers using concurrent-schedule and signal-detection procedures has found stronger support for a subtractive model over an additive model (Critchfield, Paletz, MacAleese, & Newland, 2003; de Villiers, 1980; Farley, 1980; Farley & Fantino, 1978; Lie & Alsop, 2009).
Category: