This should surprise no one. You took a large population and found subpopulations within it. If you want to look at a population average, then use the population data. If you want to look at kids with specific attention needs (guessing ADHD since medical related) then design a study to select for children fitting that criteria, including subtypes.

This seems like the type of thing that should have had a study about study design done long ago that they could have followed to help them structure their own population selection.