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 their study of the British postal population, Bentley and Haslam (1998, 2001) and Haslam and Bentley (1999) effectively highlighted that AOL occurrence resulted from combinations of factors concerning the various work situation components (organisation, physical and social environment, equipment and individual).
Category: