http://thistle.est.co.jp/tsk/detail.asp?sku=70909
【投稿】英語における文字頻度とタイプライターのキー配列 : Frequency of 2-letter Pairs in English and the QWERTY Keyboard(安岡孝一)
http://books.google.com/books?id=cK1HAAAAMAAJ&hl=ja&pgis=1
書籍名 A study of the two-letter sequences in written English used as a basis for an estimate of the efficiencies of the universal typewriter keyboard and of a suggested revision
著者 Wilson Alexander Moran
出版社 University of Wisconsin--Madison, 1930