60 Chapter 2 Beidel et al., 2021; Coles et al., 2015; Ducharme et al., 2021; Egan et al., 2021; Einfeld et al., 2018; Faja et al., 2021; Fridenson-Hayo et al., 2017; Khanna & Kendall, 2010; Kirst et al., 2022; Knox et al., 2011; Matheson et al., 2021; Matthyssens et al., 2020; Patel et al., 2006; Poppelaars et al., 2014; Poppelaars et al., 2016; Russell & Newton, 2008; Schakel et al., 2020; Shum et al., 2019; Smith et al., 2020; Sun et al., 2022; Tuijnman et al., 2022; Valenzuela et al., 2022), an applied game and casual game (Kato et al., 2008), or casual games and an active condition/treatment as usual (Dwairej et al., 2020; Fisher et al., 2015; Jivraj et al., 2020; Khanna & Kendall, 2010; Loewy et al., 2016; Wijnhoven et al., 2020). Intervention Duration Almost all of the studies that examined (sub)clinical populations included multiple sessions over multiple weeks. Only the studies of Alvares et al. (2019) and Dennis and O’Toole (2014) included participants who played one session, and in the study of Haberkamp et al. (2021) participants played the game twice a day for seven days. In studies examining anxiety in medical settings, nearly all interventions included one gameplay session which took place shortly prior and/or during the procedure. Exceptions were the study of Kassam-Adams et al. (2016) in which participants played the game as many times as they wished over a one-month period and the study of Matthyssens et al. (2020) in which participants played the game one week preoperatively. For studies measuring positive and negative affect or emotions all interventions included one gameplay session, with the exception of the study of Andrade and colleagues (2019; 2020) in which participants followed three sessions within a one-week period. The majority of interventions in studies examining mental health traits in a healthy population included multiple sessions or playing at participants’ own convenience over multiple weeks. Exceptions were the study of Tuijnman et al. (2022) in which the full program was delivered within one week and the studies of Mack et al. (2020) and Nguyen et al. (2018) in which one session per week for two weeks took place. Nonspecific Factors Results showed that nonspecific factors were not systematically measured and examined in the included studies. Only a couple of studies controlled for or measured factors such as expectations (Scholten et al., 2016; Schoneveld et al., 2018; Schoneveld et al., 2016; Wijnhoven et al., 2020), perceived helpfulness or self-reported value of the training (Bikic et al., 2017; Kuosmanen et al., 2017), therapeutic alliance (Khanna & Kendall, 2010; McCashin et al.,
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