I think to some extent reading has a binary aspect - either a kid can pick up a given piece of everyday text and read it more or less fluently, or she can't.
What most differentiates children is not their ability to decode words but to *understand* what they read. As Arthur Jensen wrote in "The g Factor",
The validity of g is most conspicuous in scholastic performance, not because g-loaded tests measure specifically what is taught in school, but because g is intrinsic to learning novel material, grasping concepts, distinctions, and meanings. The pupil's most crucial tool for scholastic learning beyond the primary grades - reading comprehension - is probably the most highly g-loaded attainment in the course of elementary education.
