159 Effectiveness of a social network intervention 5 that it can be difficult to recruit volunteers willing to commit for an extended period. To accomplish reliable evidence, more large-scale (e.g., multi-center) clinical trials collecting detailed program integrity data, intervention characteristics, and patient characteristics should be conducted; a challenging task. As discussed by researchers of a previous relatively large RCT that examined the effectiveness of a volunteer-based social network intervention, it should be considered whether an RCT design is the most appropriate method (Priebe et al., 2020). RCTs among (forensic) patients with SMI are time-consuming because these studies require long inclusion periods to obtain sufficiently large samples and a great deal of effort must be invested in data collection to avoid attrition at assessments over time. Moreover, previous studies have shown compliance problems similar to those in our study, which makes it even more difficult to achieve sufficient samples that lead to solid conclusions (McCorkle et al., 2008; Priebe et al., 2020; Siette et al., 2017). More stringent study procedures, for example, inclusion and random allocation of patients who are socially isolated and/or highly motivated to be matched to a volunteer, might raise ethical concerns. Alternatively, randomized encouragement trials, in which participants are randomly assigned to a treatment and can choose whether or not they want to receive the allocated treatment, or matched case-control studies, could be considered to increase compliance (West et al., 2008). In addition, the use of multiple methods to examine effectiveness of these complex social network interventions in forensic populations, including quantitative as well as qualitative methods, is encouraged. For quantitative outcomes, we recommend combining self-report, observer-rated, and official data sources. Further research could also include information from different perspectives, such as clinicians, romantic partners, family, and friends. Lastly, given our positive effects on duration of hospitalization and criminal behavior in outpatients, another promising line of research would be to examine the effects of the additive intervention among forensic psychiatric inpatients who may go on leave or are reentering the community after clinical treatment. Volunteers could prepare patients for reentry in the community, as shown in previous studies among reentering prisoners (Duwe, 2013, 2018b; Duwe & King, 2013). CONCLUSION There is an urgent and ongoing need to develop and improve evidence-based interventions for forensic psychiatric patients with complex needs (MacInnes &Masino, 2019; MacKenzie & Farrington, 2015; McIntosh et al., 2021; Völlm et al., 2018). This is the first study to examine whether an additive informal social network intervention could improve
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