This document discusses in depth the theoretical background of the Lexile Framework, which is based on the assumption that reading comprehension is the best predictor of success in higher education and job performance, and that it is the most tested construct in education. The first section defines measurement and differentiates between specific and general objectivity. The second section explains the Lexile Framework, and discusses: the lexile theory, the semantic component, the syntactic component, the calibration equation, the lexile scale, testing the lexile equation, interpreting lexile measures, forecasting comprehension rates, and ergonomics of the lexile framework. The third section discusses measurement error, including the components: text measure error, reader measure error, comprehension forecast error, linking standard errors, and how errors combine. Contains 8 tables, 2 figures, 3 notes, and 49 references. (EF)