In this paper, we construct a ‘telling’ case to highlight a problematic inconsistency between the results of international large-scale assessments (ILSAs) and other studies of Swedish students’ knowledge of linear equations.In this context, a ‘telling’ case, based on the scrutiny of appropriately chosen cases, is presented as a social science counter-example to the prevailing view that ILSAs’ assessments are not only valid but should underpin systemicreform. Our ‘telling’ case comparison of the different forms of study shows that Swedish students, in contrast with the summative assertions of the different ILSAs, have a secure and relational understanding of linear equationsthat persists into adulthood. We conclude with a cautionary message for the curriculum authorities.