1. Bell, A. W., O'Brien, D. J., and Shiu, C. M.: 1980, Designing Teaching in the Light of Research on Understanding, Proceedings of the 4th Annual Conference of the IGPME University of California, Berkeley.
2. Bell, A. W., Costello, J., and Kuchemann, D. E.: 1981, A Review of Research in Mathematical Education, Shell Centre for Mathematical Education, University of Nottingham. To appear.
3. CollisK. F.: 1975, Concrete and Formal Operations in School Mathematics, Australian Council for Educational Research, Melbourne.
4. Hart, K.: 1980, Secondary School Children's Understanding of Mathematics, Centre for Science Education, Chelsea College, University of London.
5. Lunzer, E. A., Bell, A. W., and Shiu, C. M.: 1976, Numbers and the World of Things, School of Education, University of Nottingham.