Quality Advisor

A free online reference for statistical process control, process capability analysis, measurement systems analysis, and control chart interpretation, and other quality metrics.

Theoretical percent out-of-spec formulas

To find theoretical % above spec:

Locate the value for Zupper in a standard normal table.

Z upper formula

To find theoretical % below spec:

Locate the value for Zlower in a standard normal table.

Z lower formula

Given:

And assuming:

In theory:

If the Z value from the standard normal table is multiplied by 100, then this process is expected to produce X% of its output outside specification.

Show me:
The standard normal table
The formulas for Target Z

Related topic:
The formula for actual percent out of spec

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