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:In this context, the effect of induced substrates on microbial communities and TCE degradation have been studied (Fan and Scow 1993; Mu and Scow 1994; Fuller et al. 1995; Shih et al. 1996; Fries et al. 1997a, b; Mars et al. 1998).
Category: