The Italian economist Vilfredo Pareto noticed that 80% of the land in Italy was owned by 20% of the population. The Pareto Principle is often referred to as the "80 – 20 rule." It’s a rule of thumb that can be applied to lots of different situations. The American quality management expert Dr Joseph Juran called it the study of the "vital few and the trivial many." 20% of something produces 80% of the results and 80% produces 20% of the results.
20% of the things you do in your life bring 80% of the results.
20% of the things that you do in the gym give you 80% of your results.
Just a few diseases account for lots of deaths.
Roughly 20% of the world’s population own 80% of the world’s wealth.
Microsoft noticed that by fixing 20% of the main reported bugs then 80% of the faults would be eliminated.
20% of the customers account for 80% of the profits.
20% of the customers give you 80% of the complaints.
20% of workers produce 80% of the output.
When I was training as a runner we did interval work. It might mean things like 12 x 2 mins, with one minute between each repetition i.e. we would run for 2 minutes hard, then jog for a minute then do the next 2 minutes hard and then a minute's jog and so on until we had completed 12 x 2 minutes of hard running. Obviously the later repetitions got harder i.e. the 9th, 10th, 11th and 12th ones in particular. However by going through the pain barrier, these last few reps would give the most long term benefit - another example of the Pareto Principle!