Data analysis
Details on the power analyses are reported in We used longitudinal multilevel models and the lme4 package in R (To test if participants showed significant changes in our outcome measures, we tested the main effects of time using multilevel models. In additional multilevel models, we tested if there was differential change between the two groups by adding a time by condition interaction term as a Level 2 predictor. Moreover, we tested if both groups were able to maintain their levels of physical activity and self-control until the follow-up assessment 4 weeks after the end of the intervention (main effect of time) and if there were differential maintenance effects between the two groups (time by condition interaction).To explore if T1 conscientiousness predicted changes in physical activity, we used a time by T1 conscientiousness interaction term as a Level 2 predictor in our multilevel models. To test for any differences between the two conditions, we used a time by T1 conscientiousness by condition interaction term as a Level 2 predictor.To test if participants who showed greater increases in self-control were better able to increase their physical activity, we added a time by change in self-control (follow-up minus pretest) interaction term as a Level 2 predictor in the multilevel models. Also, we used a time by change in self-control by condition interaction term as a Level 2 predictor to test for any group differences.