“…The majority of studies, which mostly investigated adult patients, report moderate therapist effects of approximately 5% (Johns et al, 2019 ), but the overall range of therapist effects is relatively large and varies from 0.2% to 48.7% (Crits-Christoph et al, 1991 ; Crits-Christoph & Mintz, 1991 ; Johns et al, 2019 ; Lutz, Leon, Martinovich, Lyons, & Stiles, 2007 ). Frequently researched therapist variables are the therapists’ theoretical background/orientation (Anderson, Ogles, Patterson, Lambert, & Vermeersch, 2009 ; Berglar et al, 2016 ; Okiishi et al, 2006 ), professional interpersonal skills or self-rated professional characteristics (Heinonen & Nissen-Lie, 2019 ), therapists’ alliance-building behaviours (Jungbluth & Shirk, 2009 ; Ovenstad, Ormhaug, Shirk, & Jensen, 2020 ), professional experience (e.g. years of experience doing therapy; Huppert et al, 2001 ; Turner, Nicholson, & Sanders, 2011 ), age (Anderson et al, 2009 ; Schauenburg, Dinger, & Strack, 2005 ) and gender (Shiner, Westgate, Harik, Watts, & Schnurr, 2017 ; Zorzella, Muller, & Cribbie, 2015 ).…”