Control Charts – Part 1

The wife is off to her monthly book club, the kids are in bed, and the dogs are doing what they excel at – absolutely nothing. So I thought I would settle in for a bit and begin the first of a 3 part series on one of my favorite topics – control charting.

Tonight I will explain the history of control charts, their basic characteristics, and the different types of control charts available to BOTH Lean and Six Sigma practitioners today.

Control Chart History

On a rainy day in May (OK, so I don’t know if it was raining or May but it sounds nice, eh?) a fellow by the name of Dr. Walter A. Shewhart developed what we now call control charts. He was working for Bell Telephone Labs. The year was 1924. His whole rationale for developing control charts was to study process variation, just as we should use them today.

By the way, Dr. Deming actually learned his craft from Shewhart. Sadly, it seems Dr. Shewhart gets lost in Deming’s shadows a bit. He shouldn’t. Shewhart was the man.

Control Chart Characteristics

The basic control chart has a few characteristics:

• It contains a center line that represents the mean or average of the data set
• It has an upper control limit (UCL), usually + 3 standard deviations away from the mean
• It has a lower control limit (LCL), usually – 3 standard deviations away from the mean

The reason we use +/- 3 standard deviations is that, for normal data, our data should fall within these lines 99.7% of the time when our process is in control.

When our data are motoring along between the UCL and LCL we are experiencing “common cause variation.” However, when a data point falls outside the UCL or LCL we call this a “special cause” or “signal.”

Types of Control Charts

There are few different types of control charts at our disposal. The type of data along with a few other things will help determine which chart is appropriate. Here is a basic summary:

Control charts for Attribute Data
1. p chart
2. c chart
3. u chart

Control charts for Continuous Data
4. Individuals and Moving Range chart
5. X bar R chart

I found this slick little wizard on iSixSigma which will help you choose the best control chart. You may want to bookmark it as it will serve you well.

Tomorrow night I will focus in on the p, c, and u charts so please be sure to check back then.

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