Background: The European School Survey Project on Alcohol and Other Drugs (ESPAD) is a survey study that collects comparable data on substance use of students aged 15-16 years old in European countries. The present study aims at investigating the impact of school refusal to participate in ESPAD on substance use prevalence estimates.
Methods: Data came from the 2007 German ESPAD study; the sample consisted of 12,246 students in 552 schools within seven German federal states. A simulation approach was used in order to study the effects of systematic exclusion of participating schools on prevalence estimates of key ESPAD outcomes including the use of tobacco, alcohol, cannabis, and other illegal drugs.
Results: The systematic exclusion of schools based on city-, school-, and class size, school environment, and schools' substance use policies resulted in significant changes in prevalence estimates in 23 of 25 examined combinations of selection criterion and outcome. Yet, these effects were small, with differences remaining below three percentage points around the original estimates.
Conclusions: This simulation approach suggests that nonparticipation of schools in surveys on students' substance use in Germany does not largely affect the validity of resulting prevalence estimates. Even a reduced number of schools may be sufficient to gain valid prevalence figures.