Testing anonymous link procedures for follow-up of adolescents in a school-based trial: The EU-DAP pilot study☆
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 anonymous have been proposed to partially solve issues of data protection in public registers (Quantin et al., 2000), but to date there are no examples applied to field studies involving longitudinal data.
We report on a study aiming to test accuracy and stability over time of an anonymous code self-generated by adolescents, to be used in a large trial of drug prevention in schools.
Section snippets
Methods
The Ethical Boards at the respective study centers approved the protocol of the European Drug Addiction Prevention Trial (EU-DAP study), which included field test procedures.
Accuracy of the anonymous code (AC)
Of the 222 identified forms returned by the students after anonymous code generation 39 (17.6%) carried errors, including missing digits, incorrect derivations and translocations (e.g. a character was assigned to a different digit position than expected). The proportion of erroneous codes was very different between centers. An analysis by digit determined that incorrect inputs came particularly from two particular source items, which accounted for more than 40% of the total errors: own eye
Discussion
Generating a nonsense code following strict rules at different levels (logical, visual–spatial, and typesetting) can be conceived as a task involving well developed executive functions (Anderson, 2002). Indeed a rather difficult endeavor for this geographically heterogeneous sample of adolescents. The cumulative error rate affected one out of six generated codes, which explained the rather low efficiency of the link procedure as originally planned, as just over 60% of the codes could be exactly
Conclusions
Self-generation of anonymous codes is a feasible, but not a very efficient procedure to link longitudinal data among adolescents. The use of these procedures should therefore be confined to studies with major concern for anonymity rather than for study base retention. Optimized link procedures should be employed by distinguishing between “source” and “functional” code, for example the best combination of digits from the source code that still allows for one- to-one matching.
Acknowledgments
We acknowledge all teachers and students who volunteered in this study; Samuela Bighiani, Laura Vitale and Paride Angius for their contribution to the field work in Italy, Tatiana Perez for her outstanding work at EDEX, Renée Stockling (Sweden) for editing assistance and Sara Sanchez (Sweden) for language revision.
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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).
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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).