Use the capability histogram to examine the sample observations in relation to the process requirements.
Visually examine the data in the histogram in relation to the lower and upper specification limits. Ideally, the spread of the data is narrower than the specification spread, and all the data are inside the specification limits. Data that are outside the specification limits represent nonconforming items.
To determine the actual number of nonconforming items in your process, use the results for PPM < LSL, PPM > USL, and PPM Total. For more information, go to All statistics and graphs.
Evaluate whether the process is centered between the specification limits or at the target value, if you have one. The peak of the distribution curve shows where most of the data are located.
For a parametric analysis, use Ppk to evaluate the overall capability of your process based on both the process location and the process spread. For a nonparametric analysis, use Cnpk to evaluate overall capability. Overall capability indicates the actual performance of your process that your customer experiences over time.
Generally, higher values of either statistic indicate a more capable process. Lower values indicate that your process may need improvement.
Compare statistics that measure overall capability to a benchmark value that represents the minimum value that is acceptable for your process. Many industries use a benchmark value of 1.33. If the statistic is lower than your benchmark, consider ways to improve your process.
You can assess potential capability for normal data or for data that follow a normal distribution after transformation. Use Cpk to evaluate the potential capability of your process based on both the process location and the process spread. Potential capability indicates the capability that could be achieved if process shifts and drifts were eliminated.
Generally, higher Cpk values indicate a more capable process. Lower Cpk values indicate that your process may need improvement.
Compare Cpk with a benchmark that represents the minimum value that is acceptable for your process. Many industries use a benchmark value of 1.33. If Cpk is lower than your benchmark, consider ways to improve your process, such as reducing its variation or shifting its location.
Compare Cp and Cpk. If Cp and Cpk are approximately equal, then the process is centered between the specification limits. If Cp and Cpk differ, then the process is not centered.