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:…have also been particularly well investigated (Perez-Coll et al. 1988; Pérez-Coll & Herkovits 1990; Steele et al. 1991; Strickler-Shaw & Taylor 1991; Sobotka & Rahwan 1995; Stansley & Roscoe 1996; Stansley et al. 1997; Kumar 1999; Rice et al. 2001; Rosenberg et al. 2003; Fink & Salibian 2005).
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