What is a Control Chart?

This is the start of a four-part blog on control charts. The blogs will answer the following questions:

  • What is a control chart?
  • What is variation and how does it relate to a control chart?
  • What is a control chart used for?
  • Where is a control chart used?

Control charts are at the heart of statistical process control (SPC). So maybe we should start with answering what a process is. A process is simply what we do. It can be filling out an expense report, checking a person into a hospital, driving to work, filling a prescription, etc. All these processes generate an output - either a product or service. In addition, the processes generate data. SPC is simply taking that data the process generates and using it to control and improve the process.

A control chart is used to monitor a process variable over time. That variable can be in any type of company or organization - service, manufacturing, non-profit, healthcare, etc. It provides a picture of the process variable over time and represents the way the process communicates with you. By listening to what your process is telling you through a control chart, you will know if the process is operating as it was designed to do or if the process has a problem.

Let's take the simple example of driving to work. A possible control chart is shown below.

drive to work control chart

The process variable (the time to get to work) is plotted over time. Each day, the time to get to work is measured and plotted on the chart. After sufficient points, the process average is calculated. It is plotted on the chart as shown above. Then the upper control limit (UCL) and the lower control limit (LCL) are calculated. These are also plotted on the chart.

What are the UCL and LCL? These are key to interpreting a control chart and understanding variation – the topic of our next blog.

In the meantime, if you want more detailed information on control charts, please visit our SPC Knowledge Base.

The control chart above was made using SPC for Excel, a simple but powerful software for statistical analysis in the Excel environment.

Leave a comment

Filtered HTML

  • Web page addresses and e-mail addresses turn into links automatically.
  • Allowed HTML tags: <a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd> <h1> <h2> <h3> <h4> <h5> <h6> <img> <hr> <div> <span> <strike> <b> <i> <u> <table> <tbody> <tr> <td> <th>
  • Lines and paragraphs break automatically.

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
This question is for testing whether you are a human visitor and to prevent automated spam submissions.

SPC for Excel

SPC Knowledge Base

subscribe button

Sign up for our FREE monthly publication featuring SPC techniques and other statistical topics.

Customer Stories

Click here to see what our customers say about SPC for Excel!

SPC Around the World

SPC for Excel is used in over 60 countries internationally.  Click here for a list of those countries.


Connect with Us