80-20, 20-20-60 rules   (link)

http://www.public.asu.edu/~dmuthua/pareto%27s_principle.html
Vilfredo Pareto (1848-1923) was an Italian economist who, in 1906, observed that twenty percent of the Italian people owned eighty percent of their country's accumulated wealth. Over time and through application in a variety of environments, this analytic has come to be called Pareto's Principle, the 80-20 Rule, and the "Vital Few and Trivial Many Rule." Called by whatever name, this mix of 80%-20% reminds us that the relationship between input and output is not balanced. In a management context, this rule of thumb is a useful heuristic that applies when there is a question of effectiveness versus diminishing returns on effort, expense, or time.
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When I began my librarian career as an administrator in higher education, the dean to whom I reported told me that there was a rule of thumb that had served him well. It was the 20-20-60 Rule, a special case of 80-20 Rule that he applied to a wide variety of problems and situations. His rule was that 20% of most prospects are avid supporters and 20% are avidly not supporters. The persons in these two 20% tails are basically fixed and no amount of persuasion will change their view or attitude. Prospects in the remaining 60% are persons who are interested but need to be convinced or "sold." Application of the 20-20-60 Rule means that our outcome is best if we focus on the 60% group by answering their concerns, doubts, and questions. The persons in the 60% group are the ones who most likely will become our clients and customers.