Testing anonymous link procedures for follow-up of adolescents in a school-based trial: The EU-DAP pilot study ☆ M. Rosaria Galanti a, ⁎ , Roberta Siliquini b , Luca Cuomo c , Juan Carlos Melero d , Massimiliano Panella e , Fabrizio Faggiano c,e The EU-DAP study group 1 a Stockholm Center for Public Health/Tobacco Prevention, Box 17533 118 91 Stockholm, Sweden b Department of Hygiene and Community Medicine, University of Turin, Italy c Piemonte Monitoring Center for Drug Abuse, Turin, Italy d EDEX, Bilbao, Spain e Department of Clinical and Experimental Medicine, Avogadro University, Novara, Italy Available online 18 September 2006 Abstract Objective. To study the feasibility of an anonymous coding procedure linking longitudinal information in a multi-center trial of substance abuse prevention among adolescents. Methods. A school-based survey with re-test procedure was conducted among 485 students (mean age 13.8 years) from three countries at four study centers in order to study accuracy and repeatability of a self-generated anonymous code. Results. Errors affected 18% of codes and 3% of all digits required for the code generation, with highest figures for two of the seven generation items. Sixty-one percent of the codes generated at the test were repeated identically at the re-test. Seventy-six percent of the codes could be linked excluding the 2 digits with the highest error rate in code generation, while 92% were linked using the best combination of the remaining seven or six digits. There was substantial variation between the centers in the results. Conclusions. Self-generation of anonymous codes is a feasible, but not a very efficient procedure to link longitudinal data among adolescents. Easy derivation and iterative matching procedures are crucial for achieving high efficiency of this type of anonymous linkage. © 2006 Elsevier Inc. All rights reserved. Keywords: Randomized controlled trial; Drug prevention; Follow-up; Anonymous link Introduction Longitudinal studies with repeated measurements require linking together data from the same subjects at different time points. This is usually accomplished by means of personal identifiers such as name, birth date or security number. Because of ethical considerations it is often not possible to keep identified computerized health records of young people without obtaining guardian's consent (Moolchan and Mermelstein, 2002). Although rules may vary between countries, consent procedures are often long, complex and may lead to a substantial dropout from the study base, with consequent loss of efficiency (Severson and Biglan, 1989). However, if record linking would still be possible without the use of personal identifiers, the guardian's consent would in many cases not be mandatory. Computerized procedures making personal data Preventive Medicine 44 (2007) 174 – 177 www.elsevier.com/locate/ypmed ☆ European Public Health Programme 2002 grant # SPC 2002376. National funding: Compagnia di San Paolo, grant # 2002–0703 (Novara center); Swedish Council for Working Life and Social Research grant # 2002–0979, and Stockholm County Council, Public Health grant # LS 0401–0117 (Sweden). ⁎ Corresponding author. Fax: +46 8 7373880. E-mail address: rosaria.galanti@sll.se (M.R. Galanti). 1 The EU-DAP study group: Barbara Zunino, Valeria Siliquini (Piemonte Monitoring Center for Drug Abuse, Turin, Italy); Peer Van Der Kreeft, Erwin Coppens (De Sleutel, Merelbeke, Belgium); Gudrun Wiborg (IFT-Nord, Kiel, Germany); Ann-Marie Lindahl (Stockholm Center for Public Health/Tobacco Prevention, Stockholm, Sweden);Vicky Yosidi, Clive Richardson (University Mental Health Research Institute, Athens, Greece); Maro Vassara (Pyxida, Thessaloniki, Greece); Gregor Burkhart (EMCDDA, Lisbon, Portugal); Tatiana Perez (EDEX, Bilbao, Spain). 0091-7435/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.ypmed.2006.07.019