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BACKGROUND: Universal tobacco-prevention programs targeting youths usually involve significant adults, who are assumed to be important social influences. Commitment not to use tobacco, or to quit use, as a formal contract between an adolescent and a significant adult is a preventive model that has not been widely practiced or explored and has been formally evaluated even less. In this paper, we present the rationale and protocol for the evaluation of the Swedish Tobacco-free Duo program, a multicomponent school-based program the core of which rests on a formal agreement between an adolescent and an adult. The adolescent's commitment mainly concerns avoiding the onset of any tobacco use while the adult commits to support the adolescent in staying tobacco free, being a role model by not using tobacco themselves.

OBJECTIVE: To assess (1) whether Tobacco-free Duo is superior to an education-only program in preventing smoking onset among adolescents and promoting cessation among their parents, (2) whether exposure to core components (adult-child agreement) entails more positive effects than exposure to other components, (3) the impact of the program on whole school tobacco use, (4) potential negative side effects, and (5) school-level factors related to fidelity of the program's implementation.

METHODS: A mixed-design approach was developed. First, a cluster randomized controlled trial was designed with schools randomly assigned to either the comprehensive multicomponent program or its educational component only. Primary outcome at the adolescent level was identified as not having tried tobacco during the 3-year junior high school compulsory grades (12-15 years of age). An intention-to-treat cohort-wise approach and an as-treated approach complemented with a whole school repeated cross-sectional approach was devised as analytical methods of the trial data. Second, an observational study was added in order to compare smoking incidence in the schools participating in the experiment with that of a convenience sample of schools that were not part of the experimental study. Diverse secondary outcomes at both adolescent and adult levels were also included.

RESULTS: The study was approved by the Umeå Regional Ethics Review Board (registration number 2017/255-31) in 2017. Recruitment of schools started in fall 2017 and continued until June 2018. In total, 43 schools were recruited to the experimental study, and 16 schools were recruited to the observational study. Data collection started in the fall 2018, is ongoing, and is planned to be finished in spring 2021.

CONCLUSIONS: Methodological, ethical, and practical implications of the evaluation protocol were discussed, especially the advantage of combining several sources of data, to triangulate the study questions. The results of these studies will help revise the agenda of this program as well as those of similar programs.

TRIAL REGISTRATION: International Standard Randomized Controlled Trial Number (ISRCTN) 52858080; https://doi.org/10.1186/ISRCTN52858080.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): DERR1-10.2196/21100.