Els Terlien (1947) is research fellow at Research group Numeracy. She is doing doctoral research at the University Utrecht under the name ‘Action-oriented diagnostics of mathematical knowledge and skills for young children with severe mathematics problems or dyscalculia’. The research includes the development of a reliable and valid diagnostic test for the analysis of mathematical knowledge and skills with children. The analysis leads to an individual mathematical profile of acquired mathematical concepts and solving procedures. The target group exists of children in kindergarten and grade 1-3.