Using Pareto Charts in Quality Control

A Pareto chart is a tool which helps to prioritize the issues or causes that affect a problem. The Pareto chart is based on the principle that there are usually few causes that contribute to the majority of problems. The Pareto chart is one of the most popular statistical tools used in quality control. The Pareto Chart can help to identify the major sources of defects and thus better control the quality of production.

• Use an inspection checklist (See GPG #3- Quality Control Checklists) to collect information about the frequency of defects by types or causes of defects, or assemble data that already exist.
• Identify the main causes of the defects.
• Decide what categories you will use to group items.
• Decide what measurement is appropriate. Common measurements are frequency, quantity, cost and time.
• Decide what period of time the chart will cover: One work cycle? One full day? A week?
• Based on available data, calculate the frequencies of selected types or causes of defects.
• Develop a table including 4 columns: types or causes of defects, amount or frequency, percentage of defects and accumulative amount or frequency
• Put the types or causes of defects in to the first column of the table by descending order, start with the defects with the highest frequency.
• Fill in the cells in the second column the number of defects for each type or causes of defects (by descending order)
• Calculate percentage of each types of defects by dividing number of defects of each type by total of defects for all types
• Calculate accumulative percentage and fill in the last column. The accumulative percentage in the last cells should be 100 (%).
• Calculate 80% of the accumulative percentage.
• Draw the X and Y scale and draw the column representing number of defects by descending order, start by the defect with the highest frequency
• Draw the third scale which is expanded from the last column in the chart. This is the percentage scale. The total value on the Y scale is equivalent to the 100% level of the percentage scale
• Draw the line representing the values of the accumulative percentage. The points on this line represent the cumulative percentage of each type of defects. The last point will be the same with 100% point on the percentage scale.
• The next phase is concerning with identifying the 80% point in the accumulative percentage line. Draw a vertical line from this point to the X scale. Defects represented by the column on the left of this line are the important ones. This is where the 80/20 rule comes into play, as you will most often notice that about 80% of the defective products are caused by about 20% of the possible defect causes.
• Present the Pareto chart to related people. Pareto chart can be drawn on A0 or A1 sized paper; Computer aids, such as Excel based tools are good replacement. Get comments and agreement
• Discuss the major causes of defects presented by the Pareto Chart to your working group, such as QC division of the factory. Line managers may also involve in this process. Analyze the important types or causes of defects. Are there any factors can be eliminated? Factor with highest frequency should be prioritized to be solved first.
• Discuss ways to eliminate the major sources of defects

• Line managers and quality controllers involvement
• A1 or A0 papers to draw the Pareto charts. Computer based tools with LCD projector support is a good replacement.
• Time and room for chart presentation and discussion

• Staff may feel they have more work to do
• Difficulty in collecting accurate, relevant defect information

• Help to identify the major sources of defects in a user friendly way in order to prevent them.
• Create inputs for a quality improvement program
• Saves money by helping to eliminate major sources of defects

• Pareto charts created and used in quality data analysis
• Major causes of defected identified using Pareto chart