Concern about children's mathematics performance in Ireland and elsewhere has prompted a range of responses from researchers, policymakers, educators and the media. While policy-level responses in Ireland include revising curricula and implementing a numeracy strategy that calls for increased tuition hours, teachers have also drawn on a wide array of resources in mediating curriculum-based mathematical knowledge, concepts and skills for the children in their classes. In the classroom context, formative assessment tools can be of particular value. This paper explores the potential role for one such tool, a computer-based assessment (CBA), in supporting the teaching and learning of mathematics at primary level. That test requires children to respond to open-format items, and the study illustrates how traditional error-analysis techniques are combined with the affordances of CBAs to enable automated scoring of responses. Discussion focuses on the positive role that digitally mediated error-analysis approaches can play in helping teachers understand better and address the challenges being faced by children in relation to mathematics in the middle grades of primary school. Suggestions for further research are highlighted.