Advance Innovation Group Explain about Process Capability Measure (CP and CPK)

Process capability is the ability of a process to produce desired output within specification limits given by the customers. The Process capability measure for process is done only when a process is in a state of statistical control.

CP and CPK are the two term Use to measure the capability of a process. It measures how the process is consistent around the average performance.

CP is a measure of "Process by Design" or Inherent process capability whereas CPK is a measure of Process Performance Index.

How to calculate CP:

CP is a ratio of the customer tolerance and the normal Distribution.

CP= USL – LSL

6 * std. dev

When CP = 1, Customer Specification and Normal Distribution will have to be same.

Hence the process is capable of creating products that meet customer specification.

When CP 1, Customer spec is greater than Normal Distribution.

Hence the Normal process is capable of creating products that meet customer specification.

Example of CP: Let us assume a process of having USL is 80, LSL is 20 and the Standard Deviation is 10.

If we put these values in the formula then, CP = 80 – 20 = 60 = 1

6 * 10 60

CP = 1, the Customer specification and std. dev. Is equal.

How to calculate CPK:

CPK measures the closeness of a process to its target and how consistent the process around its average performance.

CPK = Min (CPU, Cpl) while CPU = USL – Mean and Cpl = Mean – LSL

3 * std. dev 3 * std. dev

While calculating CPK we look at only one side of the distribution from the Mean, either

USL – Mean or Mean – LSL, therefore we divide by 3 std. dev. To find CPK.

CPK is the lower value from CPL or CPU.

For Example: Let us assume a process of having USL is 80, LSL is 20, Standard Deviation is 10 and mean of the process let’s say 40.

If we put these values in the formula then CPU = 80 – 40 = 40 = 1.33

3 * 10 30

Cpl = 40 – 20 = 20 = 0.666

3 * 10 30

Hence CPK will be 0.66 as it is the smaller value amongst the two values.

As the CPK value is less than 1, Comparing CPK with CP we conclude that the process is doing less what the process is capable of doing as per the process design.

Larger the CPK is better the process is performing. Generally we want a CPK of 1.33 which is (4 Sigma) or higher than that to satisfy most of our customers.

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