Abstract: This paper discusses the effect of an instructional intervention on the development of number sense of a sample of 210 Year 7 students. Before intervention, the level of number sense among the students was low. Students were highly rule bound in their approaches and were extremely inflexible in applying these rules when attempting to answer the test questions. However, after the instructional intervention, Treatment students’ number sense was significantly higher than that of the Control students. The Treatment students were beginning to use their existing knowledge of numbers and operations in more flexible ways. Several students’ misconceptions were due to learning of rules by rote and over-generalization of partially understood algorithms in solving test items. The findings will inform teachers and curriculum developers to better appreciate how students learn and think during school mathematics lessons.

Key words: Number sense; Brunei Darussalam; Mental computations; Rasch model