88 Chapter 3 To assess the magnitude of treatment effects on the primary outcome and secondary outcomes, Cohen’s f effect size will be calculated. Effect sizes of f = 0.40 are considered large, effect sizes of f = 0.25 are considered moderate and effect sizes of f = 0.10 are considered small (Cohen, 1977, 1992). Moderator analyses will be used to examine potential moderators of treatment outcome by adding potential moderators as covariates of factors to the GLMM/GEE analyses. Furthermore, multilevel mediation models will be carried out to examine potential mediators of treatment outcome. At last, appropriate statistical methodology in ATLAS.ti will be used for the analyses of the qualitative data (Boeije, 2005). DISCUSSION This article describes the study protocol of a randomized controlled trial to examine the effectiveness of the addition of an informal social network intervention (FNC) to TAU in a forensic psychiatric outpatient population. Considering the severe and multiple problems that are observed in forensic populations, more knowledge about multimodal evidence-based interventions is warranted. Social network-related problems of forensic psychiatric patients are considered one of the important targets in forensic treatment. Previous research indicates that the absence of a supportive social network can increase the risk of criminal recidivism, whereas a supportive social network can decrease the risk of criminal recidivism. Despite this, evidence-based interventions aimed at improving social networks of forensic outpatients are not available. Therefore, this RCT examines the effectiveness of the addition of FNC to TAU in improving mental wellbeing, psychiatric functioning and decreasing criminal recidivism of forensic psychiatric outpatients. We will also examine the effectiveness of the addition of FNC to TAU on several secondary outcome measures. Furthermore, this study will explore the development of a broad range of primary and secondary outcomes over time and the impact of relevant patient- and intervention-related characteristics. If effectiveness is shown, the social network intervention – based on a collaboration between formal and informal care – can be implemented more extensively within the field of forensic mental healthcare. The present study has several strengths. First, to our knowledge, this is the first RCT that examines the effectiveness of a social network intervention within a forensic outpatient population. The field of forensic (outpatient) care is in urgent need of more RCTs in order to gain knowledge about evidence-based practice (MacKenzie & Farrington, 2015; Wartna
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