Experimental design
The study adopted a cluster-randomized controlled design with two study groups. Schools were randomly assigned to either intervention or control groups. Parents and their children were assigned to either intervention group or control group respective of the children’s school. Parents in the intervention group participated in a six-week web-based need-supportive intervention program. Students were blinded to allocation, they were not informed to which study group their school was assigned.Measurements were taken in three time-points. In the pre-intervention data collection questionnaires containing baseline measures (i.e., demographics, psychological, and behavioural measures) were administered. The six-week training program was followed by post-intervention and one-month follow-up data collection occasions during which questionnaires with identical measures were administered again. In all data collection occasions, the questionnaires were distributed by researchers. The children were asked to fill in the questionnaires at home and return them in a closed envelope into the assigned box at school during the next week. Researchers collected the filled-in questionnaires from schools in 1–2 weeks after the distribution.