They used the data (1989-2008) from Basic Survey on Wage Structure (賃金構造基本統計調査) (BSWS) of the Ministry of Health, Labour and Welfare. 
Walitumia data (1989 - 2008) kutokana na Taftishi ya Msingi ya Mfumo wa Ujira (賃金構造基本統計調査) inayojulikana kama BSWS ya Wizara ya Afya, Kazi na Ustawi wa Jamii.