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: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. 1996), or both twins were part of a random sample of non-substance dependent control pairs.
Category: