Control Charts Show You Variation that Matters

Minitab Blog Editor | 29 July, 2011

Topics: Quality Improvement

They say "variety is the spice of life," but when it comes to doing business, variation is not your friend. That's why we have control charts.

We know that a little bit of variation is inevitable, but we tolerate it within acceptable limits.

When you buy a burger from a fast-food joint you want consistency, not unpredictability. Now, the pickle on your burger may be closer to the edge of the bun today than it was last week, true -- but as long as the pickle is there, it's acceptable.

Businesses use statistical process control (SPC) to keep processes stable, consistent, and predictable, so they can ensure the quality of products and services. And one of the most common and useful tools in SPC is the control chart.

The control chart shows how a process or output varies over time so you can easily distinguish between "common cause" and "special cause" variation.  Identifying different causes of variation lets you take action on a process without over-controlling it.

"Common cause" variation at our burger joint would be pickles being placed on different sides of the buns. We expect that level of variation, and it's no big deal.

"Special cause" variation would be a sudden rash of burgers that have 10 pickles instead of 1. Clearly, something unusual is causing unacceptable variation, and it needs to be addressed! 


Now you probably wouldn't need a control chart to detect special cause variation that results in 10 pickles instead of 1 on a burger. But most process variation is much more subtle, and control charts can help you see special cause variation when it isn't so obvious.

Depending on the type of data you're looking at, you can choose from several different control charts. But they all share a few basic parts:

  • Control limits: the upper and lower boundaries of variation you can accept.
  • Expected variation region: the area between your lower and upper control limits. This is where your process is expected to perform unless a change occurs. Also known as common cause variation.
  • Unexpected variation region: The area beyond your control limits. Also known as special cause variation. This variability should be investigated and acted upon.

Common Components of a Control chart

You can easily create a wide variety of different control charts using Minitab Statistical Software; in a future post I'll discuss some different types of charts and when you might want to use them.

Have you used control charts to manage your processes?