Zenkoku nanchō, gengo shōgai gakkyū jittai chōsa hōkokusho : shinshin shōgaiji no kyōiku shidō no kaizen ni kansuru chōsa fukyū jigyō / (Kokuritsu Tokushu Kyōiku Sōgō Kenkyūjo Chōkaku, Gengo Shōgai Kyōiku Kenkyūbu,, 1993)