When to use Pareto principle in our analysis?

the top 20% of the bars (depening up on your number of bins on the bar chart, usually the first 1-4 bar)on the pareto diagram shows the top 80% of the data. it is helpful in case you are looking at the number of defects vs reasons for defects. if you can chart it in the form of a pareto diagram, you can see that about 80% of the defects are being caused by about 20% of the reasons. 80-20 is an arbitary ratio. not exactly 80-20 but roughly. hope this helps.