BMC Public Health 2007, 7:322 http://www.biomedcentral.com/1471-2458/7/322 33. Hoffmann NG, Ninonuevo FG: Concurrent validation of sub- stance abusers self-reports against collateral information: percentage agreement vs. kappa vs. Yule's Y. Alcoholism: Clini- cal & Experimental Research 1994, 18:231-237. 34. Kokkevi A, Richardson C, Palermou B, Leventakou V: Reliability of drug dependents' self-reports. Drug Alcohol Depend 1997, 45:55-61. Pre-publication history The pre-publication history for this paper can be accessed here: http://www.biomedcentral.com/1471-2458/7/322/pre pub Publish with Bio Med Central and every scientist can read your work free of charge "BioMed Central will be the most significant development for disseminating the results of biomedical research in our lifetime." Sir Paul Nurse, Cancer Research UK Your research papers will be: available free of charge to the entire biomedical community peer reviewed and published immediately upon acceptance cited in PubMed and archived on PubMed Central yours — you keep the copyright Submit your manuscript here: http://www.biomedcentral.com/info/publishing_adv.asp BioMedcentral Page 8 of 8 (page number not for citation purposes)