Advance Innovation Group Define Control Charts in Six Sigma

Control charts are developed by Dr. Walter Andrew Shewhart of Bell Laboratories in the year of 1920s.

Control charts are also known as Shewhart charts or process behavior charts.

Control Charts are used to determine whether a business process or a manufacturing is performing under statistical control or not.When a process is in a state of statistical control it means the process performance will be predictable. It indicates whether a process is operating consistently or if any special cause has occurred which results in changing process mean or variance.

Control charts are bound by lower and upper control limits. If any process is moving outside these limits we can determine that our process is not in a state of statistical control. A process is called under statistical control when all the variation in the process is resulted by common cause (random) variation. Any variation in the process that occurs outside the control limits are because of special cause variation. This kind of processes are difficult to predict for future performance.

Difference between the Control limits and Specification limit

Control limits are given by process owner or by the organization but Specification limit are given by the customer.

Control limits are determined by process variation (Voice of the process) but Specification limit is determined by customer (Voice of the process).

Control limits are for average values but Specification limits are for individual value.

Types of Control charts:

Control charts for Continuous data:

There are three types of control charts that can be used when we have continuous data.

They are:

Individual & Moving Range Chart (I & MR)

X bar R Chart.

X bar S Chart.

In case of continuous data while we are using control charts this is very important to understand the number of subgroups used.

For example, if we are looking at single subgroup data like mileage of a car then we will use I & MR chart. When we have data for mileage of a car upto 5 subgroups then we will use X bar R chart and for subgroup greater than 5 we will use X bar S chart.

Note: Various practitioners have the opinion that X bar R chart is used up to 5 subgroups only and others have the opinion that it can be used up to 9 subgroups.

Control charts for discrete data:

There are four types of control charts that can be used when we have discrete data.

They are:

C chart.

U chart.

NP chart.

P chart.

Discrete Control charts can be used only for Defect data and defective data. Choice of the control chart which will be used depends on the audit sample being constant or variable.

When the data is the count of a defect and sample is fixed, we use C chart.

When the data is the count of defect and sample is variable, we use U chart.

When the data is about defective proportion and sample is fix, we use np chart.

When the data is about defective proportion and sample is variable, we use P chart.

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