The Pareto principle - 80-20 rule

Pareto Principle is the observation (not law) that most things in life are not distributed evenly.

• 20% of the input creates 80% of the result
• 20% of the workers produce 80% of the result
• 20% of the customers create 80% of the revenue
• 20% of the bugs cause 80% of the crashes
• 20% of the features cause 80% of the usage

And on and on… References:

• http://en.wikipedia.org/wiki/Pareto_principle