When Grouped by Type of Treatment What Were the Effect Sizes?The average posttest unweighted effect sizes for the five BPT studies was 0.25. The six studies had an average unweighted effect size of d 5 0.34. The five multiple component intervention studies, which combined elements of BPT, CBT, and, in some cases, other forms of support services, had an average effect size of d 5 0.90 [Singer et al., 1989; Bristol et al., 1993; Salt et al, 2002; Hudson et al., 2003; Tonge et al., 2006]. The single study of the impact of a FSI [Pelchat et al., 1999] yielded an unweighted average effect size, with mother and father effect sizes averaged, of d 5 0.37 at follow-up.