This chapter advances a definition for “college-ready data-based reasoning” and describes the development of the Critical Reasoning for College Readiness (CR4CR) Assessment. This assessment has both summative and formative elements, coordinated by an underlying learning progression. Drawing on a literature review on statistics curricula, we have developed (a) a framework for data-based reasoning at the college-entry level and (b) an instrument to measure two of the constructs in that framework: Linking Data to a Claim (LDC) and Formal Inference (FoI). The LDC construct describes student development in thinking about the link between data and the content of a statistical claim, while FoI describes student development in critical reasoning when making formal inference with respect to the notions of practical and statistical significance. Using the BEAR Assessment System (BAS) iterative approach for the instrument development, we found that the developmental progressions defined as FoI and LDC are promising, but that further iterations of the BAS cycle are needed.