CP vs CPK

In any industry, understanding the true potential of a process is vital. This will help set realistic goals and aid in avoiding unnecessary pressure to the bodies involved in the process. The demand must be realistic and it should be certain that it is viable and the process is capable of achieving it.

To measure process capability, there is a ratio or index being used and this is called as the process capability index. This measures the ability of a particular process to produce a certain output within the limits specified. Process capability is only significant for the kind of processes that can be controlled statistically. Basically, it indicates how much natural variation a process undergoes in relation to the specification limits. It also allows a better comparison between various processes.

The Capability Index

By using capability indices, one can compare an in-control process output to specification limits. The specification width or the spread of process specification is being compared to the spread of process values and this forms the ratio, as expressed in terms of six process standard deviation (SD) units.

There are two known capability indices ‘“ the Cp and Cpk. Both are used to measure process capabilities as they both compare the process spread to the specification spread. However, one is rather more accurate and gives a more clear-cut picture.

The Cp index does not take into consideration the placement of process with respect to the given limits or the specification width. In other words, it may be off-center but with cp, it doesn’t matter at all. With that being said, it is the simplest indicator of a process capability. In essence, Cp measures a process’s potential capability thus it is known as the “process potential index”.

Mathematically, it is expressed as follows:

Cp = (USL – LSL) / (6 x sigma) ;

Where:
USL = upper specification limit
LSL = lower specification limit

The drawbacks of Cp are remedied by Cpk. Both are very similar but the latter considers the centering of a particular process distribution. It can be said that it measures the variations between the target (T) and one specification ‘“ either Cpklo or Cpkhi. Cpk is also called as the “process capability index” or “process performance index”. If taken together with Cp, it will express the potential and centering of the distribution of the process within the specified limits.

Summary:

1. Cp is the simplest indicator of a process capability while Cpk gives a better picture.
2. Cp is also known as the “process potential index” while Cpk is known as “process capability index” or “process performance index”.
3. Cp index does not take into consideration the placement of process with respect to the given limits or the specification width while Cpk considers the centering of the process distribution.
4. Cp will give a description of form while Cpk provides both form and location.

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