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 absence of subsequent O1 delivery and S2 to predict the absence of subsequent O2 delivery; that is, stimuli trained in such manner exhibit properties (such as retardation) identical to those display by other conditioned inhibitors (Rescorla, 1969; Maier et al., 1976; Delamater et al., 2003).
Category: