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:Earlier work (Pan et al. 2006a) had suggested that it was necessary to raise the temperature of the organosolv pretreatment up to 195 C and increase the H2SO4 loading to 1.5% to reduce the amount of rejects in the resulting solid substrate.
Category: