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:Further, selection was based on the following criteria: (1) both twins completed the 1987 VETR enrollment survey and the 1992 Harvard Drug Study (HDS; Tsuang et al. 1996) survey; (2) at least one twin reported that he had children born between 1974 and 1988; and (3) at least one twin met criteria for a lifetime diagnoses of DSM-III-R alcohol dependence (for COA) or drug dependence (for TAP) (as determined by the HDS; Tsuang et al.
Category: