Methods: This study involved secondary data analyses from a quasi-experimental outcome study. Of the 81 girls from the original study's data set, 41 scored in the clinical range for both externalizing and internalizing disorders (comorbid group) and 40 scored in the clinical range for only externalizing disorders (noncomorbid group) on the CBCL (Achenbach, 1991) at pre-treatment. A hierarchical linear model was used to compare treatment response in externalizing scores over time between the two groups. Internalizing scores over time were examined for the comorbid group using a repeated measures ANOVA. To evaluate the clinical significance of the findings, the effect size, absolute risk reduction and numbers needed to treat were calculated.
Results: Girls with comorbid disorders experienced a significant reduction of both internalizing and externalizing symptoms. Externalizing scores were significantly different across time between the two groups, b = -1.02, p < .001. Surprisingly change for the comorbid group was steeper with a borderline large effect size in contrast to a gradual change and small effect size for the noncomorbid group for externalizing from pre- to post-treatment. A significant effect over time was detected for internalizing symptoms for the comorbid group even though treatment targeted disruptive behaviours. Only 1 in 7.4 girls, however, scored below the clinical range for both externalizing and internalizing disorders post-treatment in comparison to 1 in 5 girls scoring below the clinical range for externalizing in the noncomorbid group.
Implications: Girls with comorbid conditions benefited from treatment but the results underscore the need to target comorbidity in treatment, assessment and program design. These results also point to the need for further research on treatment development to address the serious problem of comorbidity in children's mental health and inform social work practice.