title: Nomination techniques in the study of largely invisible groups: opiate users not at drug dependence clinics.
mesh_terms: D000328:Adult; D004739:England; D005260:Female; D006126:Group Processes; D006801:Humans; D008297:Male; D009892:Opium; D019966:Substance-Related Disorders / Q000628:therapy; D013997:Time Factors
chemical_list: D009892:Opium