Researchers in both psychology and mathematics education acknowledge that children’s mathematical performance can vary inexplicably from day to day, though there has been little detailed investigation of the form of variability discussed in this paper. The paper builds on research suggesting this might be a particular issue for children considered to have learning difficulties in mathematics. The children concerned were seven- to nineyear- olds taught together for mathematics in a small group with high levels of adult help in assessment, planning and teaching. Observational research was conducted, with the researcher making weekly visits over the course of a year. Findings synthesise a range of evidence for each child gathered both during planned assessment tasks and as part of routine classroom activity. The data are used to chart the performance of individuals over this period. Findings suggest that arithmetical capacities were not fixed and easily assessed, but varied from day to day. This variability is considered in some detail with the aim of offering explanations for perceived differences. Elements such as task presentation and subtle mathematical differences between tasks provide partial explanations. Many differences remain unexplained, and it is argued that variability is in fact a feature of learning. Finally, implications for practice in assessing children and planning for their mathematical development, are considered
