Special cause variations account for about 10% of the variations in measurements. There are specific causes for these variations, they are assignable, and they must be dealt with. The other 90% or so are common cause variations.

Overview: What is variation (common cause)?

Common cause variation can be defined as the natural variation in a process. There are many causes of this type of variation, and it is difficult to remove them without re-engineering the process.

3 benefits of variation (common cause)

It would be difficult to say there are many benefits to variation as they are a hindrance to quality, but there are significant advantages of common cause variation over special cause variation:

1. Ongoing

Common cause variation is ongoing and is more seen as variation that is accepted and lived with as opposed to special cause variation which can halt processes and requires action.

2. Consistency

Common cause variation is consistent and can therefore be planned around.

3. Predictability

There is a predictability to common cause variation, which means that it does not cause any negative surprises.

Why is common cause variation important to understand?

Common cause variation is important to understand for the following reasons:

It is an inherent part of the process

It is important to understand common cause variation as just a part of the process so that no unnecessary alarm bells go off when these sorts of variations come up and a working process ends up being needlessly overhauled.


Understanding common cause variation allows you the peace of mind of knowing your process is statistically stable if the predominant variations observed are common cause.

Long-term adjustment of processes

If you understand common cause variations, you understand what kinds of variations might require immediate adjustment of processes as opposed to adjustments that should be made incrementally over the long term.

An industry example of variation (common cause)

At a newspaper, suddenly the printing press runs out of paper. This is a regular occurrence and slows down production as long as it takes to reload paper. This is an example of the type of variation that would be considered common cause and is no real need for concern or overhauling processes.

3 best practices when thinking about variation (common cause)

Here are some practices to keep in mind about common cause variation:

1. Avoid overcorrection

Many try to control common cause variation by making drastic changes and then inadvertently causing even more variation. Do not let this happen in your business. Know how much variation is acceptable and how to react to it.

2. Look at charts to determine process stability

A process is stable if the data points on a chart appear to be random and do not violate any of the 8 control chart tests.

3. Keep an eye out for variations that deviate from the mean

These are more likely to be special causes.

Frequently Asked Questions (FAQ) about common cause variation

What are some examples of common cause variation as they relate to projects?

Some examples of common cause variation as they relate to projects would be normal wear and tear, computer response times, and poor working conditions.

Why is variation seen as the enemy of quality?

Variation prohibits businesses from being able to guarantee a 100% consistent and reliable product or service.

What amount of variation is likely common cause?

Typically, if variation hovers around 2 to 3%, it is likely common cause variation.

Should You Address Common Cause Variation?

In your business, having exemplary quality control is always desired. Unfortunately, there is just some amount of common cause variation that is part of doing business that is likely out of your control. As long as you plan accordingly for it, it should not pose too big of a risk.

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