Standardized testing is generally regarded in schools today as the primary way to measure and determine student achievement. Student performance determines the degree to which schools, teachers, and students are identified as being successful. The problem being investigated by this study was that all students are spending too much classroom instructional time being assessed to predict how they will perform on annual standardized assessments (Zaleski, 2016). The purpose of this study was to compare the ability of two different diagnostic tests currently in use to predict how students would perform on the end of year state assessment for ELA and mathematics in grades three through five in a large District in south Florida. Specifically, this study analyzed the i-Ready Diagnostic and the District created interim assessment to identify the degree to which both assessments are able to predict how students will perform on the state's end of year state assessment. The relationship between the two diagnostic instruments was investigated through correlation design. The differential predictive ability of the two diagnostic assessments was explored through bivariate and multiple regression analyses as well as the calculation of sensitivity, specificity, and positive predictive values across grades (third, fourth, and fifth) and content areas (language arts and mathematics). On the surface, correlation and regression analyses suggest a high level of similarity between the two diagnostic tests. However, further exploration of sensitivity, specificity, and positive predictive values shows marked differences across the two diagnostic instruments. The pattern of differences is consistent across grades and content areas. Discussion addresses the challenges in frequent diagnostic and accountability testing and the need to make decisions that will best serve the needs of students. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]