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Control Charts

Last updated date: 21st Apr 2024
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What is the Process?

By its general term, process means a series of actions or steps that is taken in order to achieve a particular end or goal. For example, filling out an expense report, checking into a hospital or filling a prescription. Whenever we execute such actions, there is always a strong motive at the back of our head i.e., to generate an output, be it a product or service, and in the process, we also most importantly generate data. This data is beneficial to control and improve the process as it works on four dimensions: quality, quantity, timeliness and cost. It would not be wrong to say that control charts are the heart of statistical process control (SPC).

What is a Control Chart?

Control charts are also known as Shewhart charts, named after Walter A. Shewhart. It is a statistical process controlling tool which helps to monitor the improvements in the process over time. For example, hospital management decides to reduce the time taken to admit someone to the hospital and thus use a solving methodology. They start by developing the process flow diagram on how people are admitted to the hospital. Next, they measure the average time taken to admit a patient each day. And finally, plot the process variables on a control chart over time.

The objective of this control chart would be to find any "special" causes of variation as well as to reflect the process improvements that have been made. Again, to effectively use control charts, understanding the information in variation is a must.

It will be more appropriate to say that the control charts are the graphical device for Statistical Process Monitoring (SPM). Control charts perpetually monitor quality depending on the number of process characteristics to be monitored. A control chart always includes a limit such as the central line for the average, an upper line for the upper control limit, and a lower line for the lower control limit. On comparing the current data to these lines, we can draw conclusions regarding whether the process variation is in control or out of control. A control chart provides us with a picture of the process variable over time and lets us know the type of variation we are dealing with as we move forward with continuous improvement. This understanding of variation is the key to using control charts effectively.

Types of Control Charts

Since now that we fully understand what a control chart means, we shall jump directly into its types. 

There are Two Types of Control Charts: 

1) Univariate Control Chart: As we all know ‘uni’ means one, thus a, is a graphical display (chart) of one quality characteristic is called univariate control chart.

2) Multivariate Control Chart: It is also a graphical display of a statistic that summarizes or represents more than one quality characteristic.

When to Use a Control Chart?

  • A control chart can be helpful to control any ongoing processes. It finds and corrects the problem as they appear.

  • A control chart plays an outstanding role in predicting the expected range of outcomes from a process. 

  • It also helps in determining whether a process is in statistical control.

  • Control charts help to analyze the pattern of process variation from either special causes or common causes.

  • Lastly, it is of great help in determining whether the quality improvement project should target to prevent specific issues or to make fundamental changes to the process.

What are the Basic Procedures?

The basic procedures can be explained in the following steps: 

Step 1: An appropriate control chart for the data is to be selected.

Step 2: For the collection and plotting of data an appropriate time period is to be determined. 

Step 3: In this step, collection and analysis of data and construction of the chart is to be done.

Step 4: “Out of control signals” on the control chart is to be kept in mind and once it is identified, it needs to be marked on the chart and further investigation of the cause is needed. 

Step 5: Keep an account of the things learnt, the cause and how it was corrected.

Step 6: The data generated needs to be plotted and with every new data, a check for new out-of-control signals.

Step 7: In case you start a new control chart and the process may be out of control, the first 20 points calculated from the control limits will be the conditional limits. You will have to recalculate the control limits if you get at least 20 sequential points from a period when the process is operating in control.

Benefits of Process Control Charts

Control charts are used by Organizations for continuous quality improvement in the following ways:

  • Control charts provide a simple and common language for dealing with process performance and behavior.

  • It aware us with decisions about which processes to leave alone and which to subject to an improvement cycle

  • It Limits the need for inspection.

  • Control charts help to determine process capability based on past performance and trends.

  • It can also predict future performance if the system is stable and in control.

  • Control charts also assess the impact of process changes.

  • It visualizes the performance of the process over time.

  • It creates a baseline for future improvements.

  • Communicating the performance of a process is also influenced by control charts.

FAQs on Control Charts

Q1: Shouldn't Control Charts be used Everywhere?

Ans: Control charts though are helpful in many types of processes, it is not suitable for all the processes. You can never be fully confident that a control chart will always work in a specific area and will fail in the other. However, it is always advantageous to plot data over time and see what happens. You should always have an objective in mind before starting a control chart. For example, if you are working on a six sigma project and need to understand the type of variation you are dealing with. Or maybe you want to reduce a variable over time or simply just want to track a process for special causes. The point is to always have a purpose in your mind.