Summary: The aim of this study is to characterise studentsâ€™ understanding of the function-derivative relationship when learning economic concepts. To this end, we use a fuzzy metric to identify the development of economic concept understanding that is defined by the function-derivative relationship. The results indicate that the understanding of these economic concepts is linked to studentsâ€™ capacity to perform conversions and treatments between the algebraic and graphic registers of the function-derivative relationship when extracting the economic meaning of concavity/convexity in graphs of functions using the second derivative.