What are some common tools and techniques for identifying and eliminating special causes?

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Statistical Process Control (SPC) is a method of monitoring and controlling the quality and performance of a process by using statistical tools and techniques. One of the key concepts of SPC is to distinguish between common and special causes of variation in the process output. Common causes are inherent and random sources of variation that affect the process over time, while special causes are unexpected and assignable sources of variation that disrupt the process. In this article, you will learn how to identify common and special causes in SPC and what are some common tools and techniques for eliminating special causes.

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1 Why is it important to identify common and special causes?

Identifying common and special causes is important for improving the process and achieving customer satisfaction. Common causes indicate the natural variability of the process and reflect the level of quality that can be expected from the process. Special causes indicate the presence of problems or defects that need to be corrected or prevented. By identifying and eliminating special causes, you can reduce the variation in the process output, improve the process capability, and meet the customer specifications and expectations.

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2 How to identify common and special causes using control charts?

Control charts are one of the most widely used tools for identifying common and special causes in SPC. Control charts are graphs that plot the process output over time and compare it with the mean and the upper and lower control limits. The mean represents the average value of the process output, while the control limits represent the expected range of variation due to common causes. If the process output stays within the control limits, it means that the process is in control and only affected by common causes. If the process output goes beyond the control limits or shows a non-random pattern, it means that the process is out of control and affected by special causes.

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3 How to identify common and special causes using Pareto charts?

Pareto charts are another useful tool for identifying common and special causes in SPC. Pareto charts are bar graphs that rank the frequency or magnitude of different types of problems or defects in the process output. The bars are arranged in descending order from left to right, and a cumulative line is added to show the percentage of the total problems or defects accounted for by each bar. Pareto charts help to identify the most significant or frequent problems or defects that contribute to the process variation and to prioritize the actions for eliminating them. Pareto charts are based on the Pareto principle, which states that 80% of the effects come from 20% of the causes.

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Allen Scott

Management / Quality Consultant “The measure of quality, no matter what the definition of quality may be is a variable.” (Shewhart, 1931)
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Pareto has nothing to do with causes of variation. We want to know whether to leave a process to chance or find and eliminate assignable causes. The effects are measured with control charts.

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4 How to identify common and special causes using cause-and-effect diagrams?

Cause-and-effect diagrams, also known as fishbone diagrams or Ishikawa diagrams, are another helpful tool for identifying common and special causes in SPC. Cause-and-effect diagrams are diagrams that show the possible causes of a problem or defect in the process output and how they are related to each other. The problem or defect is placed at the head of the fishbone, while the main categories of possible causes are placed at the branches of the fishbone. The categories can vary depending on the process, but a common framework is to use the 6M model: materials, methods, machines, manpower, measurement, and environment. The sub-causes are then added to each category as smaller branches. Cause-and-effect diagrams help to brainstorm and analyze the potential sources of variation and to identify the root causes of the problem or defect.

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Michael Hoerl

Business Unit Quality Manager at Graham Packaging
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Cause and effect diagrams can not distinguish between common and special causes. They are a great tool to help find actionable causes to help reduce or eliminate problems.

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5 How to eliminate special causes using corrective actions?

Eliminating special causes requires taking corrective actions to address the root causes of the problem or defect and to prevent them from recurring. Corrective actions are steps or measures that are taken to fix or improve the process after identifying and verifying the special causes. Some examples of corrective actions are: adjusting or repairing the machines, changing or updating the methods or procedures, training or retraining the staff, replacing or testing the materials, calibrating or standardizing the measurement instruments, and modifying or controlling the environment. Corrective actions should be documented and monitored to ensure their effectiveness and sustainability.

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Michael Hoerl

Business Unit Quality Manager at Graham Packaging
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One thing that I have found helpful to to think about Corrective Actions as the immediate action that is taken to correct the problem. Training is almost never a true corrective action.

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6 How to prevent special causes using preventive actions?

Preventing special causes requires taking preventive actions to anticipate and avoid the potential sources of variation and to ensure the process stability and reliability. Preventive actions are steps or measures that are taken to improve the process before a problem or defect occurs or is detected. Some examples of preventive actions are: conducting regular maintenance and inspection of the machines, reviewing and updating the methods or procedures, providing continuous training and feedback to the staff, selecting and evaluating the suppliers of materials, validating and verifying the measurement instruments, and designing and optimizing the environment. Preventive actions should be planned and implemented as part of a continuous improvement cycle and a quality management system.

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What are some common tools and techniques for identifying and eliminating special causes?

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1 Why is it important to identify common and special causes?

2 How to identify common and special causes using control charts?

3 How to identify common and special causes using Pareto charts?

4 How to identify common and special causes using cause-and-effect diagrams?

5 How to eliminate special causes using corrective actions?

6 How to prevent special causes using preventive actions?

Statistical Process Control (SPC)

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