The IRI is composed of a series of graded materials, it is used to determine instructional level, and changes in reading level are recognized only when those changes affect accuracy and comprehension.
Leibert, R. E. (1981). The I R I: Relating Test Performance to Instruction--A Concept. Reading Horizons, 22 (2). Retrieved from http://scholarworks.wmich.edu/reading_horizons/vol22/iss2/6