Forty-eight studies (n=3,892) were included in the review. The mean dropout rate across studies and treatment groups was 18.1% (standard deviation 15.3). The mean duration of child-only treatments was 24.1 sessions (15.6 weeks) and the mean duration of the combined parent-child interventions was 29.6 sessions (13.8 weeks). Kappa scores for quality assessment and data extraction ranged between 0.74 and 1.00, which indicated high levels of agreement between reviewers.
The averaged weighted mean effect size was 0.27, which indicated that the addition of parent participation to child-only psychotherapy interventions added moderate improvements to treatment outcomes beyond effects attained by individual child therapy.
In the sensitivity analysis, child-treatment orientation was found to be marginally significant, which suggested that cognitive-behavioural child-only treatments provided results closer in effect to the effectiveness of all combined treatments.
The regression analyses did not show any evidence of publication bias.