What's the difference between Pp and Cp?

Process performance / capability indices describe the ability of the process to meet the specifications. When a process is not capable of meeting specifications it is worth investigating the potential changes such as centering the process average or other ways of reducing variation.

  1. On the Analyse-it ribbon tab, in the Report group, click Clone.

    The dataset worksheet and analysis task pane are displayed.

  2. In the Sigma estimator drop-down list, select Short-term.
  3. Click Calculate.
    The results are calculated and the analysis report opens.

The Cp indices are larger than the Pp indices indicating there is some variation between the subgroups that could be eliminated from the process such as more frequent recalibration of the automated colorimeter to reduce the drift in the copper concentration. Reducing the variation between subgroups and centering the process would lead to a capability ratio Cp of 1.672. Or a reduction to just 16 non-conforming units per million.

Process capability indices(click to enlarge)