Current school mathematics curricula internationally tend to emphasize problem-solving and have led to the development of opportunities for children to do maths in a more open, creative way. This has led to increased interest in performance-based assessment, which involves children in substantial production of written language to serve as evidence of their mathematical activity and achievement. However, this raises two important questions. Firstly, does this writing accurately present children's mathematical activity and ability? Secondly, do maths teachers have sufficient linguistic...