run chart vs control chart

In this blog post, we will discuss the similarities and differences between run charts and control charts. This is a request from Umasankar Natarajan, a visitor to my blog. 

These are essential tools for quality management that help identify trends and errors in the product or process. Control charts and run charts let you know: 

  • How the process is performing.
  • How today’s performance compares to yesterday.
  • Whether you are performing as planned. 

These charts are as easy to draw as they are essential communication tools for a project manager.

Let’s dive in.

Run Chart Vs Control Chart

The control chart is one of seven basic tools of quality, but run charts are not. However, the latter is equally useful in quality management.

Run Chart

run chart-1

A run chart is straightforward. It shows the variation for a single data group over time. It helps you analyze:

  • Trends in the process.
  •  If the process has shifted.
  • Whether or not the process has any patterns. 

Small to medium-sized organizations use run charts to communicate project progress with the client.

run chart-2

You will need to draw the planned progress line. Once the project starts, you will update this chart with a new line showing the actual progress. This lets the client see status and variation. 

Example of Run Chart

If you are a cricket fan, you will enjoy an example of a run chart in a live cricket match where the commentator shows you the first team’s score. Then, you’ll see the progress comparison of both teams as the second team starts batting.

Benefits of Run Charts

  • It’s easy to draft.
  • It’s easy to analyze and interpret.
  • It does not require much technical skill.
  • It is a straightforward representation of the data.

Limitations of Run Charts

  • They don’t have any statistical control limits; they don’t show you the upper and lower tolerance and threshold limits.
  • Run charts cannot show you if the process is stable and in control.

To understand a run chart, you must know the context around the data. 

Without further information, you may think a trend is normal when it is actually a variation. Sometimes, you may think a trend is abnormal when it is not.

Control Chart

While working at Bell Labs, Mr. Walter A. Shewhart developed this chart; therefore, many experts call it the Shewhart chart. It helps you study changes in the process.

control chart

A control chart is one of the seven basic tools of control, a modified version of the run chart. If you add control limits to a run chart, it will become a control chart.

Elements of Control Charts

  • Mean
  • Limits
  • Specification limits

A centerline (red) called the mean or goal is surrounded by two lines (dotted green) called limits. These are the upper control limit and lower control limit (UCL and LCL). These are surrounded by two others (purple), known as the upper and lower specification limits. 

These limits are provided in the contract and cannot be crossed. The project manager determines the upper and lower specification limits to set the process boundaries. You must take corrective action if the process crosses the set limits. 

You can say that the process is under control if 99.73% (3-sigma) of the data points fall between the upper and lower control limits. 

Control charts help you find answers to the following:

  • Is the process under control?
  • Is the project moving in the right direction?
  • Are the deliverables within the specification limits?

The Rule of Seven

You might think that instant intervention is needed when an observation is outside the control or specification limits. This is incorrect. Under one condition, you will start an investigation even if the data points are within the control limits. 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, you should investigate, even if they fall within the control limits.

Types of Control Charts

  1. c Control Charts: A c control chart helps you find non-conforming elements in mass production.
  2. p Control Charts: These control charts are useful when elements are unequal.
  3. U Control Charts: These charts help you investigate when the parameters are falling outside of the upper or lower limit.
  4. X-Bar and R Charts: These charts use the X-Bar or the mean to find subgroups. The Range plots (R Chart) the subgroups based on upper and lower control limits. These charts are useful when you need to analyze five or fewer subgroups. 
  5. X-Bar and S Charts: These charts are useful for five or more subgroups, and the standard deviations are considered in upper and lower control limits based on the mean or X-Bar.

Example of Control Charts

You can use the c control chart to see if your production process is producing the product to all specifications.

You can use a p control chart to find accidents in any particular place.

Refer to this page for more on the control chart.

Usage of Control Charts

  • To find and correct errors in an ongoing process.
  • To see if the process is stable.
  • To analyze the pattern in the process or product output.

Limitations of Control Charts

  • Control charts show common cause and special cause variations. Common cause variations are normal and usually do not require intervention, while special cause variations require attention. A control chart may show you a false special cause variation which wastes your time and resources.
  • Although control charts are easy to understand, they require knowledge of mathematical concepts like mean and standard deviation to draw the diagram. This chart requires skilled and trained personnel.
  • If the limits are placed incorrectly, you will get false indications.

Summary

Run charts and control charts are important tools in project management. A look at either can give you information that you may not get by reading reports. Both charts are handy; however, control charts provide more information than run charts. With the proper use of the former, you can eliminate errors in products and processes and focus on improving the process. 

What is your experience with the run chart and control chart? Please share through the comments section.