1. Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan
2. Office of Institutional Research, Hokkaido University, Sapporo 060-0808, Japan
3. Faculty of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan
4. National Institute of Technology, Kushiro College, Kushiro 084-0916, Japan