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56
7.1.5.4
SPC Quickie98
Cpr and Ppr
Cp Ratio or Cpr =
Pp Ratio or Ppi =
Note: Cp and Pp ratio are represented as a percentage. Cp and Pp ratio cannot be calculated if either
the LSL or USL are defined as NONE or INFINITE.
7.1.5.5
Cpi and Ppi
Cp index or Cpr =
Pp index or Ppi =
Note: Cp and Pp index cannot be calculated if either the LSL or USL are designated as a natural limit.
Cpi and Ppi will be equal to Cpk and Ppk if the process
perfectly centered between LSL and USL).
=(USL+LSL)/2 (i.e. the process average is
Cpi for a two-sided normal process is often thought of as the optimum Cpk value possible for the
process. it might be more accurately portrayed as the number of process distributions that fit inside the
specs.
7.1.5.6
Cpm
Cpm =
SUMT = (X1 - Target)2 + (X2 - Target)2 + ... +(Xm - Target)2
T
=
*AIAG Fundamental Statistical Process Control reference manual
Cpm expresses process capability in terms of each sample’s difference from “target” rather than
assuming the process should be centered between the specification limits (like CpK and PpK). As a
result, Cpm is a superior statistic to use for appraising process capability when measuring non-normal,
unilateral, or normal processes whose targets are not centered between the specification limits.
For more information about Cpm, see appendix D in Statistical Process Control Manual by AIAG or “A
New Measure for Process Capability: Cpm” Journal of Quality Technology, Vol. 20 No. 3, July, 1989.
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