Joseph Juran called it the "vital few and the useful many." A relatively small number of causes produce most of the effect. People shorten it to the 80/20 rule.
The numbers are a rule of thumb, not a law. Sometimes it's 70/30. Sometimes 90/10. The idea holds: rank first, then work the top.
The Pareto chart
ASQ counts the Pareto chart among the seven basic quality tools. It's a bar chart with the tallest bar on the left. You add a line showing the running total as a percentage. Where that line crosses 80%, you've found your vital few.
Using it on risk
Here's how our own risk tables do it:
- Score every failure mode (see FMEA for bots).
- Add up all the scores. That's the total risk.
- Sort from highest to lowest and add from the top.
- The row that carries the running total past 80% is the last one you work. Everything after it is logged as "below threshold."
The count on either side is whatever the scores produce. Don't aim for a fifth of the rows.
Severity comes first. A failure where someone could be hurt, or a law broken, gets an action whatever its score. It is never left below the line.
Below threshold isn't a sign-off. Those rows are logged, not accepted. If there's an obvious fix that costs nothing, do it. Spending more on a small item while a bigger one is open is not.
An example
| Failure | Score | Running total | % of total |
|---|---|---|---|
| Made-up figure in report | 140 | 140 | 41% |
| Wrong recipient list | 96 | 236 | 69% |
| Stale price used | 60 | 296 | 87% ← line |
| Typo in subject | 24 | 320 | 94% |
| Late by an hour | 20 | 340 | 100% |
Work the top three. Log the bottom two.
Why this helps a bot
Bots can produce long lists. A list of forty risks, all treated equally, wastes everyone's time and buries the two that matter. The 80 percent line gives the bot a stopping rule. Rank, add, draw the line, and spend the effort above it.
Where it differs from the standard method
This is our practice, not the AIAG-VDA handbook. The handbook uses an action-priority table and says an RPN threshold isn't the recommended way to decide on actions. For formal automotive work, follow the method your customer requires. The 80 percent line is a practical way to focus a design review, with severity always handled first.
Back to Engineering tools for bots.