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:Propionylations were found at 129 sites on the mid-stationary phase proteins in this study, in comparison to the 121 and 323 sites identified on midexponential and late stationary phase proteins, respectively, in our previous study (Okanishi et al. 2014).
Category: