# Sigma Level Versus Process Capability Estimates

Article Revised: March 27, 2019

From the Lean Six Sigma Green Belt Forum on 6-sigma-training.com:

## Question

I am a little bit confused about the “process sigma level calculation” :

1. The lesson presents a first method based on the RTY : First we have to compute the Normalized Yield and then derive the corresponding Z value. Finally we have to add 1.5 to account for long-term process variations and this is how we obtain the process sigma level.
2. A second method is proposed based on Process Capability Analysis. More specifically, using Zu and ZL. These values are used to evaluate the overall defective rate so that we can extract the Z value. But this time, we are not adding 1.5 to get the sigma level ? Why ?