The predominant software license is the GNU GPL. Slightly over half of the software is simply licensed using the GPL, and the software packages using the copylefting licenses (the GPL and LGPL), at least in part or as an alternative, accounted for 63% of the code. In all ways, the copylefting licenses (GPL and LGPL) are the dominant licenses in this GNU/Linux distribution. In contrast, only 0.2% of the software is public domain. http://www.dwheeler.com/sloc/redhat71-v1/redhat71sloc.html
Youll notice I find sources with a methodology listed.