Self-generated identification codes in longitudinal prevention research with adolescents: a pilot study of matched and unmatched subjects.

Self-generated identification codes (SGICs) are an increasingly utilized methodological feature of longitudinal prevention research among adolescents. This study sought to test the differences between the matched and unmatched groups at baseline on a number of background, health, and well-being and risk behavior measures in a prevention study among 13- to 16-year-old Icelandic adolescents where a SGIC was constructed and used to link individual-level respondent data over two data collection points one year apart. We use pilot data from two Reykjavik city secondary schools collected as part of the population study Youth in Iceland in February 2010 and 2011 (N = 366, SGIC matching rate 61%). Baseline results for the matched and unmatched participants are compared. Findings indicate that the unmatched subjects are both more likely to be substance users than their matched counterparts as well as being more likely to be boys and/or from disrupted families. Five out of the seven scaled measures for risk and protective factors and personality indicators reveal no difference between the matched and unmatched subjects and the significantly different measures reveal small effect sizes between the two groups. However, the effect sizes for substance use are significantly different between the matched and unmatched groups for all seven substance use measures with effect sizes from 0.52 to 1.32. These findings therefore indicate that the measurement validity of adolescent risk behaviors such as substance use may be put in jeopardy when using SGIC and that unmatched subjects may be more likely to distrust the SGIC process.