Testing anonymous link procedures for follow-up of adolescents in a school-based trial: The EU-DAP pilot study

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. (c) 2006 Elsevier Inc. All rights reserved.