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 coloured microspheres were quantitated by extracting the dye into a standard volume of dimethylformamide (DMF) and by making use of the linear relationship between the absorption of the dye solution and the number of spheres (Kowallik et al. 1991).
Category: