This article argues that a "probabilistic" interpretation of competence can provide the basis for a link between assessment, teaching and learning, curriculum resources and policy development. Competence is regarded as a way of interpreting the quality of performance in a coherent series of hierarchical tasks. The work of Glaser is combined with that of Rasch and Vygotsky. When assessment performance is reported in terms of competence levels, the score is simply a code for a level of development and helps to indicate Vygotsky's zone of proximal development where the student is ready to learn.