Concordance and consistency of answers to the self-delivered ESPAD questionnaire on use of psychoactive substances.

Considering the prevalence of drug use in Italy, it is crucial to develop a reproducible screening test. Test-retest reliability and internal consistency are important indicators of a measurement's temporal stability and are a necessary condition for validity. The aim of the study was to assess the consistency and concordance of the European School Survey Project on Alcohol and Other Drugs (ESPAD) questionnaire; participating students completed the questionnaire twice, with a three-week interval. To verify the concordance for variables relating to use of alcohol, cigarettes and illicit drugs, the original ordinal variables as well as the same dichotomically recodified variables were used. Data analysis was done using Kappa and weighted Kappa. The method proposed by Lipsitz was used to evaluate the influence of gender and age on concordance. Questions about drug use, examined in ordinal form, show a good test-retest concordance and an excellent concordance for answers relating to the use of cigarettes, alcohol and cannabis. Regarding the effect of age adjusted for gender, 15-year-old subjects showed a lower concordance than 19-year-olds. ESPAD is a tool with a good reproducibility. Results focus on the effect of gender and age covariates on the concordance of answers regarding drug use and suggest the importance of examining the concordance in relation to the covariate levels.