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 results support the conclusion that an increased mortality from lung cancer was observed among workers initially employed before 1955 in two soluble/mixed beryllium facilities (Lorain and Reading), but not among beryllium workers at any plant in 1955 or later, or those who worked at facilities that only processed insoluble forms of beryllium materials during any period [15, 16].
Category: