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:Spectra for water and the O2 dimer are not included in the DOAS fit as previous retrievals found 15 their inclusion unnecessary (Boersma et al., 2007; Bucsela et al., 2006).
Category: