Current college admissions tests inadequately predict college success (Geiser & Studley, 2002) and provide insufficient information for students and teachers regarding college readiness at earlier stages of students' academic careers (Olson, 2006). This article reports on the creation and validation of a more informative test rooted in college readiness standards in mathematics. Items and scoring guides were created using mixed-methods of assessment design, and assessment results were analyzed and validated using item response theory, student interviews, and teacher evaluations of test items. This research shows how an innovative measurement approach can be used for more accurately assessing college readiness, and reveals one possible means of providing better measures along a central strand of mathematical knowledge required for students entering college. (Contains 5 figures, 3 tables and 8 footnotes.)