A list of the shiftwords or shiftgrams in some word stock, classified by the amount of shift, such as Leonard Gordon's "Letter-shift Words in the OSPD" (WW 3.1990-61), shows that some shifts produce more word pairs than others. To get some insight as to why, this article surveys the whole stock of shiftwords and shiftgrams of words of lengths 3-12 from a specified word stock, before going on to analyze words of some specified lengths in further depth.
"Shiftgrams Ciphered, Analyzed,"
Word Ways: Vol. 36
, Article 4.
Available at: http://digitalcommons.butler.edu/wordways/vol36/iss4/4