This will be a LINKS-PAGE, with summaries (of the main ideas) plus links to pages where you can learn more.
The Pareto Principle – Descriptions, Explanations, Applications
link from Higher Chronicle - http://www.bsu.edu/libraries/ahafner/awh-th-math-pareto.html
Chronicle - http://chronicle.com/blogs/profhacker/the-80-20-principle/31626
I.O.U. — There will be more links here later.
The 80/20 is estimated not calculated, and it's approximate not exact. Therefore, don't interpret the 80/20 literally.
The 80/20 simply means that "usually, not all things are equal in value." Sometimes instead of 80/20 an estimated split should be 90/10, or 70/30, or (when values are equal, or at least similar enough) 50/50.
Sometimes "80 + 20 = 100" when you split a whole (= 100%) into categories. But... the "best 20%" may not produce 80% of results; instead it might be 25%, 40%, or 90%.