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:The Index was developed by Zill and Peterson, based on numerous preexisting instruments (Achenbach & Edelbrock, 1981; Graham & Rutter, 1968a; Kellam, Branch, Agorwal, & Ensminger, 1975; Langner et al., 1976; Rutter, Tizard, & Whitmore, 1970; National Center for Health Statistics, 1971).
Category: