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:Details of the sampling, field, and laboratory procedures have been described previously (Hinck et al., 2006, 2007a,b, 2008a; Schmitt, 2002; Schmitt et al., 2005).
Category: