Control Chart versus Run Chart

By Fahad Usmani 

Today I’m going to discuss the control chart and run chart upon a request from Umasankar Natarajan, who is a visitor to my blog and asked me to write on the seven quality control tools.
So I decided to start with control charts. However, before studying the control chart, it is necessary to understand the run chart, which is similar to the control chart and often misunderstood with it.
Control charts and run charts are important tools in quality management, which help you identify trends or errors in the product or the process. A short look at these charts can give you answers to many questions, such as:

• How is the process performing?
• How is today’s performance vs yesterday’s performance?
• Are we doing good or bad?

These charts are easy to draw and can be an important communication tool for the project manager.
Okay, let’s get started.

Run Chart

A run chart is the simplest chart of all. It shows the variation in a single data group over a period of time. A run chart helps you analyze the following:

• Trends in the process; i.e. whether the process is moving upward or downward.
• Trends in the output of the manufacturing process.
• If the process has any cycle or any shift.
• If the process has any non-random pattern in behavior over a period of time.

Run charts are widely used by small to medium sized organizations, because they help the project manager communicate the actual progress of the project with the planned progress to the client.

In this type of chart, the project manager will draw a chart showing the planned project’s progress from beginning to the end on monthly basis. Once the project starts, he will update this chart with the actual progress and show it to the client, so that the client is able to see the variation in the planned and actual progress.
If you are a cricket fan, then you will see the example of a run chart in a live cricket match telecast where commentator shows you the score progress of the first team. Then when the second team starts batting, he will show you the comparison of the performance of both teams through the run chart.

Benefits of Run Charts

The following are a few benefits of a run chart:

• Easy to draft.
• Easy to analyze and interpret.
• Does not require much technical skill.
• Straightforward representation of data.

Limitations of Run Charts

The drawbacks of run charts are that they don’t have any statistical control limits. Simply put, they don’t show you the upper and lower tolerance and threshold limits.
Run charts are not capable of showing you if the process is stable and in control. Moreover to understand the charts, you need to be well aware of the context and situation in which data has been collected and the chart is drawn. For example, you may think that some trend exists in the graph when in fact you are just seeing a normal process variation. This can also explain the opposite, i.e. there is a trend in the graph and you think it is normal.

Control Chart

A control chart is one of the seven basic tools of quality control. It is an updated version of the run chart. If you add control limits to the run chart, it will become a control chart. Control chart is also known as Shewhart chart, because it was developed by Walter A. Shewhart while working in Bell labs. These charts are used to study the change in a process over a period of time.

Elements of Control Charts

In control chart, there is a center line (red line) called the mean or goal which is surrounded by other lines (dotted green lines) called limits. These lines are the upper control limit and lower control limit (UCL and LCL). These lines are again surrounded by two another lines (purple lines) known as the upper specification limit and lower specification limit.
Upper and lower specification limits are provided in the contract and you cannot cross them. This is your final limit. The upper and lower control limit are determined by the project manager so that specific limits are not crossed, and if the process goes above this limit, a corrective action must be taken.
If the 99.73% (3-sigma) of all the points fall between the upper and lower control limits, you will say that the process is under control.
Control charts help you find answers to questions such as:

• Is the process under control?
• Are you going in the correct direction?
• Are the products within the specification limits?

Rule of Seven

You might think that if you have any reading either outside the control limits or specification limits, then only the process requires an intervention, and in all other cases there will be no investigation. However, you’re wrong. There is one special case where although all data points lie within the control limits, it requires an investigation. This case is known as the “Rule of Seven”.


This rule says that if seven or more consecutive data points fall on one side of the mean, then there must be an investigation initiated to find out the reason for it, even if these points fall within the control limits.

Usage of Control Charts

You can use control charts in the following cases:

• When you want to find and correct an error in an ongoing process.
• When you want to see if the process is in control or not.
• When you want to analyze the pattern in process or product output.

Limitations of Control Charts

The following are a few limitations of a control chart:

• The control charts designed to provide you with common cause variation and special cause variation. Common cause variations are considered to be normal and usually do not require any intervention. On the other hand, special cause variation requires attention from the project manager. A control chart may sometimes show you a false special cause variation, causing wastage of your time and resources.
• Although control charts are easy to understand, they do require knowledge of some mathematical concepts, such as mean and standard deviation, to draw the diagram. If your organization lacks a skilled person to draw the control chart, you may face problems.
• If the limits are placed incorrectly, you may get false indications.

Summary

A picture is worth a thousand words. A simple look at these charts can give you a lot of information that you may not get by reading a bunch of reports. In project management, both charts are very useful; however, control charts provide more information than run charts.
With the proper use of control charts you can eliminate errors in product and process and focus your attention on improving the process.
I hope you have enjoyed reading this blog post. In this blog post, I have covered the control chart, the rest of the seven quality control tools will be discussed in forthcoming posts.
Here is where this blog post on control and run charts ends. If you have something to share, you can do so through the comments section.
image credit => Sale Graph/FreeDigitalPhotos.net