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 models are the radial-modesof-vibration model of Abdullah (1966), the tornado/ boundary interaction model of Tatom et al. (1995), and the corotating vortex model (Powell 1964; Georges 1976; Mitchell et al. 1992).
Category: