In this article, we report on a design experiment conducted in an 8th grade classroom that focused on students' analysis of bivariate data. Our immediate goal is to document both the actual learning trajectory of the classroom community and the diversity in the students' reasoning as they participated in the classroom mathematical practices that constituted this trajectory. On a broader level, we also focus on the learning of the research team by documenting the conjectures about the students' statistical learning and the means of supporting it that the research team generated, tested, and revised on-line while the experiment was in progress. In the final part of the article, we synthesize the results of this learning by proposing a revised learning trajectory that can inform design and instruction in other classrooms. In doing so, we make a contribution to the cumulative development of a domain-specific instructional theory for statistical data analysis.Our purpose in this article is to report on a 14-week classroom design experiment conducted with a group of eighth-grade students that focused on statistical covariation. In presenting the analysis, we focus on both the trajectory of the students' learning and our own learning about the means of supporting the students' learning. This latter issue, the learning of the research team, is central to the design experiment methodology, in that conjectures about the trajectory of students' learning are tested and modified on-line while experimenting in a Requests for reprints should be sent to Paul A. Cobb,