This document describes the development and application of a model for generating a core reading vocabulary screening test based on rank frequency distributions derived from a quarter of a million word corpus of 500 word samples embracing commerce, English, home economics, industrial education, mathematics, science, and social studies. The word selection algorithm is based on probabilistic concepts drawn from work in information science and keyboard indexing and uses frequency data to partition vocabulary lists into sections ranging from high frequency to rare words. Word types selected from these sections constitute general vocabulary, general subject vocabulary, and special subject vocabulary. A high utility 496 word vocabulary list is presented for use in screening both native speakers and second language students who will have difficulty in reading texts assigned in junior secondary subject areas. The word list is cross validated using frequency data drawn from the Kucera and Francis adult word list and the American Heritage elementary list. Suggestions for calibrating and using the core vocabulary test within the emerging framework of criterion referenced testing are also included. (Author)