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:This is in comparison to previous work (Pan et al., 2006a) using a one-stage organosolv pretreatment of poplar where 90% of the original cellulose was recovered in the final water insoluble fraction.
Category: