Reference:
Baldwin SA, Murray DM, Shadish WR, Pals SL, Holland JM, Abramowitz JS, Andersson G, Atkins DC, Carlbring P, Carroll KM, Christensen A, Eddington KM, Ehlers A, Feaster DJ, Keijsers GP, Koch E, Kuyken W, Lange A, Lincoln T, Stephens RS, Taylor S, Trepka C, Watson J. Intraclass correlation associated with therapists: estimates and applications in planning psychotherapy research. Cognitive Behaviour Therapy 2011;40(1): 15-33.