Also called the 80/20 rule. The principle that about 80 per cent of the outcomes will come from about 20 per cent of the effort. Commonly, the Pareto Principle is the acceptance that most things in life are not distributed fairly or evenly. An example of using the Pareto Principle would be to say that:

20% of input creates 80% of the result or 20% of the work force produce 80% of the result.

See also:
Cost–benefit analysis
Cost-effectiveness league table (healthcare economics)