Pareto analysis for quality problems

01The principle

Pareto analysis ranks categories (defect types, causes, suppliers, part numbers) by frequency or impact and shows the cumulative percentage. In many processes a small number of categories account for most occurrences, often summarised as the 80/20 principle, although the exact ratio varies.

02How to build a Pareto chart

1. Define the categories clearly and collect data over a representative period.
2. Count occurrences or total cost per category.
3. Sort in descending order and calculate the cumulative percentage.
4. Plot bars and the cumulative line; mark the 80% level.

Use the Pareto chart generator to do this from your own data.

03Count, or weigh by cost?

Frequency alone can mislead. Ten cosmetic scratches cost less than one cracked casting. Weight categories by cost or severity when deciding where to act, and show both views when presenting to management.

04Stratify

A single Pareto can hide patterns. Stratify by supplier, shift, machine or product family. A defect that dominates one supplier may be absent elsewhere, which is valuable information for root cause analysis.

05Example: casting rejections

The chart below shows an illustrative distribution of casting rejects found after machining. Gas porosity and shrinkage together exceed 60% of rejects, so foundry process improvements should start there.