The most recently developed model to be discussed (Huitt, 1995) identifies the major categories of variables that have been related to school achievement. The model is not only school-, classroom-, teacher-, and student-based, but includes additional contextual influences as well. Huitt's model attempts to categorize and organize all the variables that might be used to answer the question, "Why do some students learn more than other students?" This is a revision of a model by Squires, Huitt and Segars (1983) which focused only on those variables thought to be under the control of educators. This earlier model focused on school- and classroom-level processes that predicted school learning as measured on standardized tests of basic skills. One important addition in this model is the redefinition of Academic Learning Time. It had long been recognized that Carroll's conceptualization of time spent measured the quantity of time engaged in academics, but was lacking in terms of the quality of that time. As discussed in Proctor's (1984) model, Fisher and his colleagues (1978) had added the concept of success as an important component of quality of time spent and coined the term Academic Learning Time (ALT) which they defined as "engaged in academic learning at a high success rate." Brady, Clinton, Sweeney, Peterson, & Poynor (1977) added another quality component--the extent to which content covered in the classroom overlaps to content tested--which they called content overlap. Squires et al. used the more inclusive definition of ALT proposed by Caldwell, Huitt & Graeber (1982)--"the amount of time students are successfully engaged on content that will be tested."