http://blog.livedoor.jp/fristunderstand/archives/44683576.html

http://www.huffingtonpost.jp/toru-kumagai/productivity-of-labor_b_7198038.html

http://www.chosunonline.com/site/data/html_dir/2015/05/09/2015050900472.html

http://www.nikkan-gendai.com/articles/view/news/159648

http://www.nishinippon.co.jp/nnp/syasetu/article/167688