“…Evidence of the effectiveness of group-based behaviour-change interventions (GB-BCIs) is accumulating through systematic reviews and meta-analyses. For example, GB-BCIs have been found effective in smoking cessation (Stead & Lancaster, 2009;West, Walia, Hyder, Shahab, & Michie, 2010), promoting physical activity (Harden et al, 2015) and walking (Hanson & Jones, 2015), diabetes self-management (Deakin, McShane, Cade, & Williams, 2009;Steinsbekk, Rygg, Lisulo, Rise, & Fretheim, 2012), self-management among cancer patients (Smith-Turchyn, Morgan, & Richardson, 2016), and weight loss (Borek, Abraham, Greaves, & Tarrant, 2018). Despite encouraging evidence, GB-BCIs exhibit substantial, largely unexplained cross-intervention heterogeneity in effectiveness and considerable variability in design (e.g.…”