The Pareto principle - the 80/20 rule

In the late 1800s, Vilfred Pareto observed that 80% of income in Italy was received by 20% of the Italian population. The Pareto law, in its generalized form, states that 80 % of the objectives - or more generally the effects - are achieved with 20 % of the means - or more generally the causes or the agents. 80% of effects coming from 20% of causes.