The Pp-Index compares the tolerances of the process (how it should be) to the process spread (how it is). The long-term standard deviation sLT  is calculated using long term data, usually 25-50 samples within a longer time frame representing all process variation – i.e. months.

Calculation:

Pp = (USL – LSL) / 6sLT

Assumptions:

  • Data are representative of the process
  • The process is stable (no special causes)
  • Data are normally distributed

Attention:

  • In contrast to the Ppk index, the Pp index does not consider the center of the process! For the example below, the Cp indices of process A and process B are the same. However, process A is obviously better than process B as there are fewer defective parts.
  • In contrast to the Cp Index, the Pp index is used to describe the capability of the process considering long term variation.

Read also: Cpk, Process Capability