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:Differences may result in part due to the fact that BIOLOG selects for only a portion of the microbial community (Smalla et al., 1998) while PLFA analysis includes the whole microbial community, regardless of activity.
Category: