This study examined prospective secondary mathematics teachers' professional noticing of students' reasoning about mean and variability as described in a current statistics curriculum in the USA. Six prospective mathematics teachers were asked to analyse students' written solutions to statistics problems, constructed at the two levels described in the statistics curriculum. Findings indicated that prospective teachers had difficulties noticing students' reasoning about variability. None of the prospective teachers explicitly interpreted the student's limited understanding of variability when comparing data sets with unequal sample sizes. There was a gradation in their noticing skill, ranging from those who showed no evidence of differentiating between students' different levels of reasoning and made general pedagogical decisions to those who differentiated between students' developmental levels and decided how to respond based on students' existing understanding as well as identifying the intrinsic statistical properties in students' solutions. Implications for research and mathematics teacher education are discussed.