values, which is not surprising because a certain level of effortful control is necessary for students to be able to persevere through the task of completing all measures, year after year. Also, a higher number of missing values was related to higher levels of tobacco use in Grade 11. Last, boys had significantly more missing data than girls did. All the significant correlations between the number of missing values and the previously mentioned variables were in the small range, that is, r ≤ 0.15.
