Year 11 (15-year-old) students are not exposed to formal statistical inferential methods. When drawing conclusions from data, their reasoning must be based mainly on looking at graph representations. Therefore, a challenge for research is to understand the nature and type of informal inferential reasoning used by students. In this paper two studies are reported. The first study reports on the development of a model for a teacher’s reasoning when drawing informal inferences from the comparison of box plots. Using this model, the second study investigates the type of reasoning her students displayed in response to an assessment task. The resultant analysis produced a conjectured hierarchical model for students’ reasoning. The implications of the findings for instruction are discussed.
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