A Pareto chart is a graph that helps you see which categories contribute the most to a total problem. It combines bars sorted from largest to smallest with a line showing the cumulative percentage. This makes it useful for finding the “vital few” causes that deserve the most attention.
In quality control, business, engineering, and school projects, Pareto charts help turn raw counts into clear priorities.
The bars show category frequencies, such as defects, delays, complaints, errors, returns, setup issues, and miscellaneous causes. The cumulative percentage line adds each category’s contribution as you move from left to right. If the first few categories account for most of the total, the chart supports the 80-20 idea, meaning a small number of causes may create a large share of the effects.
A Pareto chart does not prove cause and effect, but it helps teams choose where to investigate and act first.
Key Facts
- A Pareto chart uses bars sorted from highest frequency to lowest frequency.
- Cumulative count after category k = sum of counts from category 1 through category k.
- Cumulative percentage = cumulative count / total count x 100%.
- The cumulative percentage line usually starts above the first bar and ends at 100%.
- The 80-20 idea suggests that about 80% of effects may come from about 20% of causes.
- Pareto charts are best for categorical data, such as defect types, complaint reasons, or error sources.
Vocabulary
- Pareto chart
- A graph that displays categories in descending order with bars and adds a cumulative percentage line.
- Frequency
- The number of times a category or event occurs in a data set.
- Cumulative percentage
- The running percent of the total after adding each category from left to right.
- 80-20 rule
- The idea that a large share of outcomes often comes from a small share of causes.
- Category
- A group or label used to organize data, such as defects, delays, or returns.
Common Mistakes to Avoid
- Leaving the bars unsorted is wrong because a Pareto chart depends on ranking categories from largest to smallest to show priorities clearly.
- Using percentages without checking the total is wrong because each category’s percent must be based on the same overall total.
- Treating the 80-20 rule as exact is wrong because it is a guideline, not a law that must always produce exactly 80% and 20%.
- Assuming the biggest bar is the root cause is wrong because a Pareto chart shows frequency, not proof of why the problem happens.
Practice Questions
- 1 A company records defect counts of 45, 30, 15, 6, and 4 for five defect types already sorted from largest to smallest. What is the cumulative percentage after the first two categories?
- 2 A service desk has 50 delays, 25 complaints, 15 errors, 5 returns, and 5 setup issues. Make the cumulative percentage values for the categories in this order.
- 3 A Pareto chart shows that complaints are frequent but inexpensive, while returns are less frequent but very costly. Explain why a team might need more than the Pareto chart before choosing what to fix first.