Pareto Principle (The 80/20 Rule)

Originally, the Pareto Principle referred to the observation that 80% of Italy’s wealth belonged to only 20% of the population.
More generally, the Pareto Principle is the observation (not law) that most things in life are not distributed evenly. It can mean all of the following things:

• 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…

But be careful when using this idea! First, there’s a common misconception that the numbers 20 and 80 must add to 100 — they don’t!
20% of the workers could create 10% of the result. Or 50%. Or 80%. Or 99%, or even 100%. Think about it — in a group of 100 workers, 20 could do all the work while the other 80 goof off. In that case, 20% of the workers did 100% of the work. Remember that the 80/20 rule is a rough guide about typical distributions.
Also recognize that the numbers don’t have to be “20%” and “80%” exactly. The key point is that most things in life (effort, reward, output) are not distributed evenly – some contribute more than others.