Example of Nonparametric Capability Analysis

A team of engineers wants to evaluate the capability of their paper bleaching process. The engineers randomly collect 200 samples of pulp and record the brightness level. The lower specification limit for the brightness level is 72. The engineers want to assess capability without making any assumptions about the distribution of the data. The engineers perform nonparametric capability analysis to evaluate how well the process meets customer requirements.

  1. Open the sample data, PulpBrightness.MTW.
  2. Choose Stat > Quality Tools > Capability Analysis > Nonparametric.
  3. In Single column, enter Brightness.
  4. In Lower spec, enter 72.
  5. Click OK.

Interpret the results

Some of the 200 samples are below the lower specification limit of 72. The capability index Cnp is missing because the process uses one specification limit. The capability indices Cnpk and Cnpl are equal because the process has a lower specification limit. Because Cnp and Cnpk are comparable to parametric capability indices like Cp and Cpk, the team assesses the capability of the process relative to familiar guidelines. The Cnpk is 0.89, which is less than the generally accepted industry guideline of 1.33. Therefore, the engineers conclude that the process is not capable and does not meet customer requirements.