4. Draw the specification limits on the distribution
Draw vertical lines on the distribution to represent the lower and upper specification limits. In the example, the lower specification limit (LSL) is 0 minutes (on time) and the upper specification limit (USL) is 14 minutes. Estimate where the two lines should be located in reference to the overall average and the tails of the curve. Label each specification with its abbreviation and value. The example completed through this step follows.
The diagram shows whether any portion of the curve is beyond the specifications. In the example, some of the distribution is beyond the upper specification. If the overall average of the distribution is outside the specification, refer to “Variation – Capability analysis where the overall average is outside the specification” later in this section.
The above article is an excerpt from the "Operational definition" chapter of Practical Tools for Continuous Improvement Volume 2 Statistical Tools.