In this blog post, we will discuss the control chart and the run chart. This is a request from Umasankar Natarajan, who is a visitor to my blog and asked me to write about the seven basic quality control tools.
Although a run chart is not one of these basic quality control tools, knowing it will help you understand the control chart.
Control charts and run charts are essential tools in quality management that help you identify trends or errors in the product or the process. These charts let you know:
- How the process is performing.
- Today’s vs. yesterday’s performance.
- Whether you are performing as planned.
These charts are easy to draw and are essential communication tools for a project manager.
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 non-random patterns.
Small to medium-sized organizations use run charts to communicate the project’s progress with the client.
For this chart, you will draw the planned progress line. Once the project starts, you will update this chart with a new line showing the actual progress, so that the client can see the status and variation in the planned and actual progress.
For example, if you are a cricket fan, you will see an example of a run chart in a live cricket match where the commentator shows you the score of the first team. Then, they will show you the progress comparison of both teams via a run chart when the second team starts batting.
Benefits of Run Charts
The following are a few benefits of a run chart:
- 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
The following are a few drawbacks of a run chart:
- 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 should know the context in which the data was collected.
For example, you may think a trend is normal, while in fact, it was a variation. Sometimes, you may feel a trend is abnormal when it is not.
Mr. Walter A. Shewhart developed this chart while working at Bell Labs; many experts call it the Shewhart chart. It helps you study changes in the process.
A control chart is one of the seven basic tools of quality control and is 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
A control chart has the following elements:
- Specification limits
A centerline (red line) called the mean or goal is surrounded by others (dotted green lines) called limits. These lines are the upper control limit and lower control limit (UCL and LCL). These are again surrounded by two others (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. These limits are determined by the project manager to set the process boundaries, and if the process goes beyond them, corrective action will be taken.
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 questions:
- Is the process under control?
- Is the project moving in the right direction?
- Are the deliverables within the specification limits?
Rule of Seven
You might think that intervention is required the moment an observation is outside the control limits 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 these points fall within the control limits.
Usage of Control Charts
You can use control charts in the following cases:
- To find and correct an error 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
The following are a few limitations of a control chart:
- 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, wasting your time and resources.
- Although control charts are easy to understand, they require knowledge of mathematical concepts, such as mean and standard deviation, to draw the diagram. This chart requires skilled and trained personnel.
- If the limits are placed incorrectly, you may get false indications.
Run charts and control charts are important tools in project management. A look at either of these 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.
Do you use these charts in your project? If yes, please share how you find them useful in the comments section.