4.5. Calculate how much data is outside the specifications
As indicated in the previous step, some of the distribution is outside the specification limit. The question is, how much? To determine the percentage that falls outside the specification limits, it is necessary to find how many estimated standard deviations exist between the overall average and each specification limit. The number of standard deviations is known as the Z value. Z values are used to determine the percentage of output that is outside the specification limits using the standard normal distribution table.
The above article is an excerpt from the "Operational definition" chapter of Practical Tools for Continuous Improvement Volume 2 Statistical Tools.