This research investigates the question of what growth, if any, is shown by teachers in identifying the components of children's reasoning using an upper and lower bounds argument for a fraction task. Specifically, it reports on assessment outcomes from design-based research in teacher education that measures teachers' identification of children's reasoning from studying videos. We describe the nature of the instructional intervention as well as the video-based assessment used as a pre and post measures for identifying children's mathematical reasoning, and report on the nature of teacher growth in recognizing components of children's arguments. [For the complete proceedings, see ED584829.]