The middle years of schooling (Grades 5 to 9; ages 10 to 14 years), critical in students’ mathematical development, are also characterised by a dip in mathematics performance. Typically, the greatest drop occurs at the transition between primary and high school. StatSmart is a large-scale longitudinal study addressing statistics teaching and learning in the middle years in three states in Australia, one of which has Grade 7 as part of the primary school, whereas in the other two states Grade 7 marks the start of high school. Student understanding of statistics is measured three times using Rasch measurement to compare performances at different points in time. Two cohort measures indicated that there was a plateau in performance between the first and second year of high school, rather than at the primary/high school transition. The trajectories of learning identified are discussed.