From the abstract: "This paper presents and describes an instrument called the Assessment of Student Knowledge of Variability (ASK.Var) that measures students' progress in learning to think about data aggregations and variability. Through use of a Rasch model to iteratively refine the instrument, we have developed a measurement tool that correlates well with results from an open-response assessment that asks students to justify reasoning about data and variability." Good sample objective assessment at the end of the paper.