Sources of Variation

Section 2 Variation Fundamentals

Lesson S02-08

Text: Section 2 pages 59 – 67

Duration: 22 minutes

There are two primary objectives in this section. The first is to identify the sources of variation. Once we know the sources of variation, the largest contributors to the output can be determined (with the assistance of other statistical methods).

The second objective is to learn whether each source of variation is a natural and expected source of variation or whether the source can lead to unexpected outcomes in the output.

Common cause sources of variation are sources of variation that influence the process in an expected or predictable manner. In other words, if we observe a stable process, we conclude that the relevant sources of variation are common cause sources. “In control” processes are subject to common cause sources of variation only by definition.

Special cause sources of variation are factors that cause unexpected changes to the process. Phrased differently, special causes imply an “out of control” condition. They may affect the process average by causing a shift in the location. They might also cause the amount of variability in the process to increase or decrease significantly.