We should say at the outset that the complexity
of some of the work we will describe in this article
was necessitated in large part by the fact that
it is difficult to tease apart the unique contribution
that reading volume affords. One of the difficulties
is that levels of reading volume are correlated
with many other cognitive and behavioral
characteristics. Avid readers tend to be different
from nonreaders on a wide variety of cognitive
skills, behavioral habits, and background variables
(Guthrie, Schafer, & Hutchinson, 1991;
Kaestle, 1991; Zill & Winglee, 1990). Attributing
any particular outcome to reading volume is thus
extremely difficult.