Pareto Principle

aka Pareto's Law. Management consultant Joseph Juran developed the concept in the context of quality control, and improvement, naming it after Italian economist Vilfredo Pareto, who noted the 80/20 connection while at the University of Lausanne in 1896.[4] In his first work, Cours d'économie politique, Pareto showed that approximately 80% of the land in Italy was owned by 20% of the population. The Pareto principle is only tangentially related to Pareto efficiency. http://en.wikipedia.org/wiki/Pareto_principle

aka 80/20 rule