This study continues our research to further validate the idea that ADHD (Attention Deficit/Hyperactivity Disorder) interferes with transition from one task to another and this interference can be quantified by a Consistency Index (CI) derived from a specific mathematical representation of EEG data. We reanalyze 32 previously reported data sets present new data for 35 boys and girls, ages 7-12, ADHD or control. Each data set contains EEG, recorded and digitized while participants perform consecutive 10-min tasks: video, reading, and math. For boys, the CI in ADHD was four times lower than in controls, p < .005, for girls this difference was two times, p < .05. ADHD/control classification based on the CI coincided with the DSM-IV criteria for 88% of the boys and for 67% of the girls. Post hoc analysis indicated that the classification utility of the CI diminished with age. A CI below 40% could be a discriminating, reliable, and reproducible marker of ADHD in young boys.