The lower specification limit (LSL) of the process is the minimum allowed value for the product or service. This limit does not indicate how the process is performing but how you want it to perform. You specify LSL when you set up the capability analysis.
If you use a data transformation when you perform the capability analysis, Minitab also calculates LSL*, which is the lower specification limit for the transformed data.
Use the USL and LSL to define customer requirements and to evaluate whether your process produces items that meet the requirements.
The upper and lower specification limits are identified by the vertical dashed lines on the histogram. Compare the histogram bars to the lines to assess whether the measurements are within the specification limits.
The specification spread is the distance between the upper specification limit and the lower specification limit (USL – LSL). Suppose a company produces ball point pens and the target outer diameter for the ball is 0.35 mm. The acceptable range for ball outer diameters is 0.34 to 0.36 mm. Therefore, the LSL is 0.34, the USL is 0.36, and the specification spread is 0.02 mm.
Minitab compares the specification spread to the process spread to determine your process capability.