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:Immunofluorescence microscopy, colony and Western immunoblotting, and determination of switch frequencies were carried out using anti-α%$ antiserum in accordance with established procedures (Eisenstein, 1981; Caffrey et al., 1988; Henderson et al., 1997a).
Category: