End products are open.This type of problem has multiple correct answers.As stated above, Shimada and his colleagues have been developed this type of problems (e.g., Shimada, 1977). In Europe, Christansen & Walter (1986) talked about the importance of investigation problems, which is similar to the problems that the end products are open. An example of this type, "marble problem," is shown below, which is well known as a representative problem in the open-ended approach.