LCL and UCL basically represents the -3 sigma and +3 sigma respectively. The PMBOK phrase seemed to denote 3 different SD measurements, but it was indeed referring to the 3times sigma value on both sides of mean.

Thanks for the clarifications!

Chan,

Please watch episode E08.04 Advanced Quality Management Tools where we discuss these control limits. And the standard deviation is explained in E08.00 Project Quality Management Overview

Hello Everyone,

I found the following definition of Control limits in the PMBOK glossary.

"The area composed of three standard deviations on either side of the centerline, or mean, of a normal distribution of data plotted on a control chart that reflects the expected variation in the data."

What does the three standard deviations denote? The two control limits UCL and LCL are shown in the control chart diagram. What is the third SD mentioned here?