Methods: Data come from students recruited in 5th grade from 30 elementary schools in Phoenix and followed through the end of 8th grade. Schools were assigned to treatment or control conditions through block randomization, adjusting school size and ethnicity. Mexican American students were in the majority in the study schools, 82 percent of all the participants. This analysis utilizes the subsample of 1,670 Mexican Americans. After obtaining active parent consent for 83% of enrolled 5th graders, students completed self-administered questionnaires at baseline (Fall 2004), prior to any intervention, and 5 post-tests: Spring 2005, Spring 2006 (6th grade), Fall 2006 (prior to implementation of the 7th grade intervention), Spring 2007, and Spring 2008 (8th grade). Students were tracked: the proportion providing data declined from 91% at the first post-test to 74%, 60%, 55%, and 47% at the last wave. Treatment effects were estimated using growth curve models of changes in the frequency of last 30 day use of four substances (alcohol, cigarettes, marijuana, inhalants), adjusting for school-level nested data, and multiply imputing missing data due to attrition. Models entered dummy variables for treatment groups (5th only, 7th only, 5th and 7th), with the control group as reference, along with demographic predictors (gender, age, academic performance, and SES), and quadratic terms modeling possible curvilinearity in the substance use trajectories.
Results: Compared to controls, the 7th grade only intervention significantly reduced the growth trajectory of use of all four substances. Those receiving the intervention both in 5th and 7th grade showed desired realtive reduction for marijuana and inhalants only. Students receiving the intervention only in 5th grade were statistically indistinguishable from controls on all outcomes.
Conclusions/Implications: There was clear evidence that intervention only in middle school is more effective than intervention in elementary school alone. For two substances—inhalants and marijuana—students reported less steep increases in use when intervention occurred in 7th grade, whether or not there was an earlier 5th grade intervention. Implications include the possibility that prevention messages may be less effective when delivered before peak ages of experimentation—in middle school—or before youth have developed better decision-making skills. For alcohol and cigarettes—substances most readily available to preadolescents—intervening in 5th grade not only appeared ineffective, but may have compromised the preventive effects of intervening later, in 7th grade. Curvilinear growth curves for inhalants and marijuana suggested that the 7th grade intervention accelerated a developmental leveling off in the frequency of their use.