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:Although the density of staining in the cerebellar molecular layer appears similar for all 3 ligands, the number of binding sites for 3H-forskolin is approximately IO-fold lower than for either 3H-IP, or )H-PDBu, which are comparable (Worley et al., 1986b, 1987a).
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