Difference Between Cp and Cpk

A process capability measures the potential of a process. That is its ability to produce a product within set limits.

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Cp and Cpk are such process capability indices.

Cp vs Cpk

The difference between Cp and Cpk is that Cp analyzes the ability of a process to justify the structured specification for a product. While Cpk portrays the deviation of a process from the center within the tolerance range. Cp is termed as Process Capability. Cpk is Process Capability Index. 

Cp vs Cpk

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 is the efficiency of the process.

Comparison Table

Parameters of ComparisonCpCpk
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)/6Cpk = (Xdbar- LSL)/3          ORCpk = (USL-Xdbar)/3
The minimum value (for a capable process)1.0 – 1.332.00

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 centered 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.

Condition for a perfectly centered process is 

Cp = Cpk .

Main Differences Between Cp and Cpk

  1. Cp is the difference between USL and LSL divided by 6. Cpk equals to (Xdbar- LSL)/3. or  (USL-Xdbar)/3.Here USL is Upper Specification Limit. LSL is Lower Specification Limit.
  2. Cp must be at least 1, Cpk must have a minimum value of 2

References

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