ENCAL: a prototype computer-based learning environment for teaching calculator representations. - Andrew G. Harrop (PPIG, 1999)

The Design of ENCAL, a Learning Environment Utilising Multiple Linked Representations. - Andrew G. Harrop (Intelligent Tutoring Systems, 2002)

Teaching Children Brackets by Manipulating Trees: Is Easier Harder? - Thomas R. G. Green; Andrew G. Harrop; Vania Dimitrova (Diagrams, 2004)