In education, a curriculum (/kəˈrɪkjʉləm/; plural: curricula /kəˈrɪkjʉlə/ or curriculums) is broadly defined as the totality of student experiences that occur in the educational process. The term often refers specifically to a planned sequence of instruction, or to a view of the student's experiences in terms of the educator's or school's instructional goals. In a 2003 study by Reys, Reys, Lapan, Holliday and Wasman they refer to curriculum as a set of learning goals articulated across grades that outline the intended mathematics content and process goals at particular points in time throughout the K-12 school program. Curriculum may incorporate the planned interaction of pupils with instructional content, materials, resources, and processes for evaluating the attainment of educational objectives.. Curriculum is split into several categories, the explicit, the implicit (including the hidden), the excluded and the extra-curricular Curricula may be tightly standardized, or may include a high level of instructor or learner autonomy. Many countries have national curricula in primary and secondary education, such as the United Kingdom's National Curriculum.UNESCO's International Bureau of Education has the primary mission of studying curricula and their implementation worldwide.