Six Sigma For Dummies, 2nd Edition
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Besides control chart points that lie beyond the control limits in Six Sigma, other visual patterns can tell you that something out of the ordinary is happening to your process. These other patterns also indicate special cause variation.

Detecting special cause patterns, shifts, and drifts in a control chart is similar to detecting out-of-the-ordinary behavior in a pair of dice. The probability of rolling a 7 with two dice is 6 in 36, or about 17 percent. That’s because you have 6 possible ways to roll a 7 with two dice, out of a total of 36 possible outcomes.

What is the probability of rolling a 7 twice in a row? The combined probability is 17 percent (0.17) multiplied by 17 percent (0.17), or 2.8 percent (0.028). The probability of rolling a 7 three times in a row is 0.17 × 0.17 × 0.17, or about 0.46 percent.

So if you see someone roll a 7 three times in a row, that probability is small enough that you can safely conclude something out of the ordinary must be going on. You use this same thinking to detect patterns, trends, and shifts in control charts.

Dividing the distance between the control limits and the process average into three equal zones, you can use the following rules to detect special causes of variation:

  • Any one point beyond either control limit

  • Two out of any three consecutive points in Zone A, and all three on the same side of the process average

  • Four out of any five consecutive points in Zone B or A, and all five on the same side of the process average

  • Fifteen points in a row in Zone C, on either side of the process average

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About This Article

This article is from the book:

About the book authors:

Craig Gygi is Executive VP of Operations at MasterControl, a leading company providing software and services for best practices in automating and connecting every stage of quality/regulatory compliance, through the entire product life cycle. He is an operations executive and internationally recognized Lean Six Sigma thought leader and practitioner. Bruce Williams is Vice President of Pegasystems, the world leader in business process management. He is a leading speaker and presenter on business and technology trends, and is co-author of Six Sigma Workbook for Dummies, Process Intelligence for Dummies, BPM Basics for Dummies and The Intelligent Guide to Enterprise BPM. Neil DeCarlo was President of DeCarlo Communications.

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