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:Additionally, the studies in general point out that the type of transportation seems to be more related to social and environmental factors than to the students’ freedom of choice, as observed in the studies carried out by Davison et al. (2008); Dollman & Lewis (2007); Robertson-Wilson et al.
Category: