A process capability measures the potential of a process. That is its ability to produce a product within set limits.
Cp and Cpk are such process capability indices.
Key Takeaways
- CP (Process Capability) measures a process’s potential performance, while CPK (Process Capability Index) evaluates actual performance against specification limits.
- CP only considers the process’s natural variation, while CPK also considers process centering and alignment with target values.
- CPK is more informative for process improvement, as it reflects both the potential capability and how well the process meets specifications.
Cp vs Cpk
Cp means process capability, and it is the capacity of a process to follow the strategic classification for a product. It is equal to or greater than the Cpk. Cpk stands for process capability index and estimates the variation of a process from the inside of the tolerance range and is less than the Cp.
Cp, also known as Process capability, is the statistical calculation of a process’s ability to produce a product within a given set limit. It is the ratio of the total specification to the total effective range.
The capability of a process is proportional to its magnitude. The higher the CPK, the higher the efficiency of the process.
Comparison Table
| Parameters of Comparison | Cp | Cpk |
|---|---|---|
| Name | Process Capability. | Process Capability Index. |
| Definition | Cp is the capacity of the process to abide by the strategic classification for a product. | Cpk estimates the variation of the process from the center inside the tolerance range. |
| Magnitude | Cp is either equal to Cpk or greater than Cpk. | Cpk is always less than Cp. |
| Formula | Cp = (USL-LSL)/6 | Cpk = (Xdbar- LSL)/3 ORCpk = (USL-Xdbar)/3 |
| The minimum value (for a capable process) | 1.0 – 1.33 | 2.00 |
What is Cp?
Cp is one of the process capability indices. It is called Process Capability. Process capability (Cp) is a statistical evaluation of a process’s capability to produce products within a determined limit.
Cp is the ratio of the ‘voice of the customer’ to the ‘voice of the process. It depicts the range of the process and lets us judge its effectiveness.
Evaluation of Cp
Cp = Total specification (or Tolerance)/ Total effective range (or natural variability)
Cp = (USL-LSL)/6, here USL = Upper Specification Limit; LSL = Lower Specification Limit; = standard deviation.
For instance, if you are preparing a canvas, its width and breadth must be considered. To be precise, products must have a specific measurement or structure.
What is Cpk?
Cpk is the process capability index. It is the measure of how well a process is centred within the specified limit. It implies the capability of a process.
It depicts the localization of the process as well. Cpk is the minimum of Cpl, Cpu. Cpl and Cpu are process potential (one-sided).
Cpl = – LSL/3, Cpu = USL-/3. Here is the target of the process. A good process has a larger Cpk. Negative Cpk means the process has exceeded the given specification.
The condition for a perfectly centered process is
Cp = Cpk.
Main Differences Between Cp and Cpk
- Cp is the difference between USL and LSL divided by 6. Cpk equals to (Xdbar- LSL)/3. or (USL-Xdbar)/3.Here, USL is the Upper Specification Limit. LSL is a Lower Specification Limit.
- Cp must be at least 1, Cpk must have a minimum value of 2
- https://onlinelibrary.wiley.com/doi/abs/10.1002/qre.990
- https://www.cps.org.tw/issues/file/201412192253180.pdf
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