Twenty-six studies (999 participants) were included: 19 studies (all drug-treatment studies) used a crossover design and 7 studies used a between-patients parallel design.
The methodological quality scores ranged from 0 to 5 with 17 studies scoring at least 3. There was substantial heterogeneity among the studies analysed with respect to quality, design, sample size, patients' characteristics, interventions, outcome measures, and follow-up.
Medication versus placebo.
The overall SMD using the teacher rating scales (18 studies, 1,317 participants; some participants counted twice on account of the study designs) was -1.028 (95% confidence interval, CI: -1.212, -0.843; chi-squared 35.33, d.f.=17).
The overall SMD using the parent rating scales (13 studies, 1,135 participants; some participants counted twice on account of the study designs) was -0.859 (95% CI: -1.140, -0.578; chi-squared 52.58, d.f.=12).
Psychological or behavioural therapy versus control or no comparison.
The overall SMD using the teacher rating scales (2 studies, 50 participants) was -0.398 (95% CI: -1.276, +0.480; chi-squared 2.37, d.f.=1).
The overall SMD using the parent rating scales (1 study, 26 participants) was -0.488 (95% CI: -1.270, -0.294).
Combination therapy (medication plus behavioural therapy) versus control or comparison.
The overall WMD using the teacher rating scales (2 studies, 36 participants) was -3.777 (95% CI: -8.064, +0.510; chi-squared 1.96, d.f.=1).
The overall WMD using the parent rating scales (2 studies, 36 participants) was -7.345 (95% CI: -12.289, -2.401; chi-squared 1.63, d.f.=1).
Combination therapy versus medication.
The overall WMD using the teacher rating scales (3 studies, 76 participants) was 1.285 (95% CI: -0.717, +3.286; chi-squared 1.15, d.f.=2).
The overall WMD using the parent rating scales (2 studies, 38 participants) was -0.460 (95% CI: -3.861, +2.942; chi-squared 0.30, d.f.=1).
Combination therapy versus psychological or behavioural therapy.
The overall WMD using the teacher rating scales (3 studies, 68 participants) was -2.006 (95% CI: -4.174, +0.163; chi-squared 1.44, d.f.=2).
The overall WMD using the parent rating scales (2 studies, 39 participants) was -5.911 (95% CI: -8.631, -3.190; chi-squared 0.22, d.f.=1).