Effectiveness of Multisystemic Therapy for Adolescents From Families With Intellectual Disabilities

The goal of this observational study is to determine the effectiveness of a specialisation of multisystemic therapy (MST) for adolescents with severe behavioural problems from families with an intellectual disability (ID; MST-ID). To achieve this goal, a mixed method study design is used. To this end, a quantitative and a qualitatively primary research question are formulated: * Is MST-ID superior, when compared to standard MST, in reducing rule-breaking behaviour of adolescents (quantitative)? * What are the experiences of adolescents and/or parents receiving MST-ID treatment (qualitative)? Participants will be asked to complete two screeners (questionnaires delivered as a verbal interview) with a total duration of approximately 30 minutes. Other data will be collected through Routine Outcome Monitoring questionnaires that are part of standard MST procedures. To this end, five 'time points' have been identified: T0 (start of MST\[-ID\] treatment), T1 (end of MST\[-ID\] treatment), T2 (follow-up 6 month after MST\[-ID\] treatment), T3 (follow-up 12 month after MST\[-ID\] treatment), and T4 (follow-up 18 month after MST\[-ID\] treatment). The qualitative method used to gain insight into families' experiences is determined in consultation with the families. To assess the effectiveness of MST-ID, its treatment outcomes will be compared to standard MST treatment outcomes of families with ID.