Pareto chart

A Pareto chart is a bar chart ordered from most frequent to least frequent, with a cumulative percentage line on a second axis. It applies the 80/20 rule (Pareto principle): roughly 80% of problems come from 20% of causes. The chart makes the “vital few” causes visible at a glance.

How to build a Pareto chart

1. Count the frequency of each defect type or category.
2. Sort categories from highest to lowest frequency.
3. Calculate the total.
4. Calculate cumulative frequency and cumulative percentage for each bar.
5. Draw bars (left axis = count) and a cumulative line (right axis = %).

Worked example

A paint line logs defects over one month:

Total defects = 84. Scratches and dents together account for 71.4% of all defects. A team that eliminates just two root causes—scratches and dents—would expect a 71% reduction in defect count. That is the power of the Pareto approach.

Practice problem

Five failure codes: A=30, B=12, C=8, D=6, E=4. Total=60. What cumulative percentage do A and B represent together?

Common exam traps

• Always sort by frequency, not alphabetically. The most-frequent category must be the first (tallest) bar on the left.
• “Other” always goes last. Even if the “other” category has more entries than the last named category, it is placed at the right end of the chart because it is a catch-all, not a specific root cause.
• The 80/20 rule is a guideline, not a law. Real data might show 60/30 or 90/10. The chart is useful regardless; focus on the categories to the left of the 80% line.
• Pareto charts show frequency, not impact. The highest bar is the most frequent problem, not necessarily the most costly. Use a weighted Pareto (count × cost per defect) when cost matters more than volume.

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