The capability plot is located at the lower right corner of the Normal Capability Sixpack Report.
The capability plot consists of three intervals:
- The within interval represents the potential process tolerance and is calculated by six times the within-subgroup standard deviation.
- The overall interval represents the actual process tolerance and is calculated by six times the overall standard deviation.
- The specification interval represents the interval between the lower and upper specification limits (LSL and USL.
The process center is shown by the tick mark inside the Overall and Within process spread intervals. The target, if you have one, is shown by the tick mark inside the specification spread (Specs).
Use the capability plot to visually assess the capability of your process.
To assess the potential capability of your process, compare the within process spread with the specification spread. To assess the overall capability of your process, compare the overall process spread with the specification spread. If the process spread is greater than the specification spread, your process needs improvement.
Compare the process center to the target, if you have one. If the process is centered (on target), the middle tick marks for the specifications and for the process spread are vertically aligned. If the process is not centered, investigate the reasons why.
If the within process spread and the overall process spread differ greatly, your process may not be stable. Check the control charts to ensure that your process is in control.