This study analyses the influence of trainers' intrapersonal and group management competences on the effectiveness of the Universal Strengthening Families Program 11-14 (SFP 11-14). More specifically, it assesses the effect of these competences on internalizing and externalizing symptoms in adolescents. The analysed data is made up of ratings given by the 174 mothers participating in SFP 11-14. The results confirm the effectiveness of SFP 11-14 in reducing internalizing and externalizing symptoms in adolescents. Using linear regression models, evidence is provided of the influence of the trainers' expertise, in terms of their competences, in improving internalizing symptoms in adolescents (through a reduction in levels of anxiety, depression, and somatization and in the global internalization scale). Emphasis is placed on how trainer competences can impact on the effectiveness of evidence-based programmes, stressing that this should be taken into account by the public authorities and other stakeholders in the assessment and design of family evidence-based programmes.
Keywords: externalizing symptoms; family-based programmes; internalizing symptoms; the influence of trainers; trainer competences.