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