Six Sigma Measure Phase in the Six Sigma approach to problem solving requires us to understand the extent of process variation. You will learn about the process variation in Lean Six Sigma Green Belt training, and in some of our Six Sigma courses for free. It tells you how many defects a process will cause. A process must have as few defects as possible to be efficient. Therefore, it must also have minimal process variation. We need to visualize process variation in the Measure Phase of DMAIC to assess where we stand with regard to process performance.
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The Measure Phase Control Chart is one way to see process variation. The 7 QC tools include the Measure Phase Control Chart and other control charts. The Measure Phase Control Chart shows the outputs of a process over time. It also displays how the process performs against control limitations that are set to match the maximum or minimum variation in a process to meet customer demands.
What is Process Variation?
Before we can move on to the Measure Phase Control Chart, it is important to first understand the concept Process Variation within the context of this Measure Phase Control Chart. There are two types of Process Variation. Every process is subject to variation.
Common Cause Variation (or CCV): CCV is caused by natural process variation. It is the result of the design of the process. CCV is simply a result of many, ever-present variations in the process. CCV can be illustrated by the variations that may be observed when there are multiple people involved in the process. Each person may do things differently. SCV refers to variation due to non-random or assignable causes.
Special Cause Variation (SCV): This is a result of causes that are not usually present in the process. A special cause variation might be the variation that occurs when an untrained person is allowed to work in the process. Special cause variation can also refer to the process variation that can be attributed a reason.

Determine Special Cause Process Variation
The Measure Phase Control Chart is the best way to determine if the variation was caused by Common Cause or Special Cause. The Measure Phase Control Chart is also known as a time series plot and is used to monitor a process over time. It is a plot of the process characteristic, usually over time, with statistically defined limits. It is used to monitor process and help the user decide the best course of action.
The History of the Measure Phase Control Diagram
In the 1920s, Dr. Walter A. Shewhart introduced control charts to the general public. Hewhart Control Charts are a collection of control charts that monitor a single quality characteristic.
Control Limits in a Measure Phase Chart
A Measure Phase Control Chart is also a representation of the frequency distribution over time in the order that the data occurred. It also adds three reference lines to help interpret patterns in the data. These reference lines include a center line and two controls limits. The Measure Phase Control Chart’s center line is the average or mean. Two control limits are known as the Upper and Lower Control Limits. The Measure Phase Control Chart’s control limits are defined as three-sigma either side of the data’s mean.
This begs the question: Why are there three standard deviations? It balances the possibility of a false signal while maintaining sensitivity to detect real signals. The signs of Special Cause Process Variation include points that fall below or above the lower control limit, trends or runs, and any other unusual patterns. To determine if the process has changed, it is worth investigating any indications of a special reason.

Process Variation: What?