LSL
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.
Note
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.
Interpretation
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.
USL
The upper specification limit (USL) of the process is the maximum 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 USL when you set up the capability analysis.
Note
If you use a data transformation when you perform the capability analysis, Minitab also calculates USL*, which is the upper specification limit for the transformed data.
Interpretation
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.
Target
The target is the ideal value of a process based on your customer requirements. For example, if a cylindrical part produces optimal performance in a product when the diameter is 32 mm, then 32 mm is the target for this part.
Note
If you use a data transformation when you perform the capability analysis, Minitab also calculates Target*, which is the process target for the transformed data.
Interpretation
Use the target to define optimal process performance and to compare with the mean of your process.
The target value is usually, but not always, centered between the lower and upper specification limits. When you have a target, examine whether your process is centered near the target.
Minitab uses the target to calculate Cpm, which describes the capability of your process relative to the target and the specification limits.
