Looking at the "21 foot rule" - Again. From Blake Consulting
Cop in the Hood
EDGED WEAPON DEFENSE: Is or was the 21-foot Rule Valid? An article attributed to the Force Science Institute
21 feet - what does it really mean? This is an article on Linked In and you may have to have a Linked In account to access it.
There is also references in the PERF report referenced at this link to the 21-Feet rule.