The following description is taken from an affidavit by Gerald R. McMenamin, one of the leading scholars in the field; the affidavit – from Case 1:10-cv-00569-RJA -LGF Document 50 Filed 06/02/11.
As part of his expert witness statement, McMenamin describes the theoretical and practical foundation of the method by which he examined documents and determined that they were not written by Facebook founder Mark Zuckerman. See http://www.nytimes.com/2011/07/24/opinion/sunday/24gray.html .
These are the principles I apply to my authorship analyses.
Principles of stylistic analysis