Mathematical knowledge is critical for teachers before they can help students to learn. Ball (1988b) remarked that "knowledge of mathematics is obviously fundamental to being able to help someone else learn it" (p. 12). The importance of teachers' mathematical knowledge for the improvement of mathematics teaching and learning was further advocated by Ball, Lubienski, and Mewborn (2001). Content knowledge does influence the decisions teachers made about classroom instruction. When a teacher has a conceptual understanding of mathematics, it influences classroom instruction in a positive way (Fennema & Franke, 1992). Thus, it is essential that teachers need to competent in their subject matter knowledge (SMK) in order to help their students learn mathematics meaningfully.
Studies on teachers' SMK had been primarily determined by the number of mathematics content courses the teachers had taken and passed, or by reviewing their results from mathematics competency tests (Ball, 1991a, 1991b). Baturo & Nason (1996) had classified this as teachers' substantive knowledge which focused mainly on the correctness of the subject matter. However, teachers who tried to help their students to learn required more than substantive knowledge which includes teacher's understanding of the nature and discourse of the discipline (Ball, 1991b; Ball & McDiarmid, 1990; McDiarmid, Ball, & Anderson, 1989).