Maree, J., Basson, N., & Malan, R. (2001). Optimising learning competencies of grade 9 learners: A comparison between a few subject didactic measuring instruments. Suid-Afrikaans Tydskrif Vir Natuurwetenskap En Tegnologie / <i>South African Journal of Science and Technology</I&gt;, 20(3/4), 80-93. https://doi.org/10.4102/satnt.v20i3/4.255