The purpose of this paper is to report the research that took place during the development of the "ENRIGHT Diagnostic Inventory of Basic Arithmetic Skills." This is a criterion-referenced test designed to provide educators with information useful for changing instructional strategies based on known error patterns of students in grades 1 through 9. The Inventory is a group- or individually-administered 3-level system developed to minimize testing time. Each level narrows the area of interest until the critical skill is reached. The overall Inventory measures 144 sequentially ordered skills. This report describes the development procedure, which involved testing and studying approximately 7000 students over a 5-year period. The curriculum referencing component as well as the content validity, and subtest reliability studies conducted during the research are also described. The study as a whole led to the development of 233 error patterns that are descriptive of all predictable errors found in the research. The paper concludes with the descriptions of the three forms of information provided by the inventory: (1) computational skills competency; (2) analysis of error patterns; and (3) grade level information. (YP)