Intervention compared with control families reported a greater decrease in ADHD symptoms at three and six months ( adjusted mean difference for change in symptom severity -2.9 , 95 % confidence interval -5.5 to -0.3 , P = 0.03 , effect size -0.3 , and -3.7 , -6.1 to -1.2 , P = 0.004 , effect size -0.4 , respectively ) .