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:During compression, the AmB molecules change their orientation to vertical at a surface pressure of approximately 15 mN/m (Arczewska and Gagoś 2011; Gagoś and Arczewska 2010; Gruszecki et al. 2002).
Category: