Developing a diagnostic instrument designed to disentangle student reasoning~from~conceptual understanding*

A common overarching goal of physics instruction is to help students improve the conceptual understanding and reasoning necessary to analyze basic physics situations successfully.~However, existing physics assessments tend to focus on improvements in \textit{overall} student performance, and do not readily help instructors ascertain whether observed performance issues primarily stem from reasoning difficulties or conceptual difficulties (or both). As such, our collaborative multi-institutional research group has been developing a two-tier instrument, the \textit{Dimensions of Conceptual Understanding and Reasoning Instrument (DCURI),} designed to disentangle the two dimensions of productive thinking (conceptual understanding and reasoning). The instrument allows instructors to diagnose the likely source(s) of poor student performance, which, in turn, can inform instructional modifications. The instrument also allows for an assessment of improvements along each of the two dimensions separately. As a result, instructors, researchers, and curriculum developers not only can examine overall improvement in student performance but also can pinpoint the impact of instructional interventions on each dimension.