How to Calculate Cp and Cpk

Cp stands for process capability and Cpk stands for process capability index. Both are used for the measure of a potential capability of a process in short term. The higher the sigma level, the better the process is performing. You can learn how to calculate Cp and Cpk values using this tutorial. Learn to calculate the Process Capability (Cp) and Process Capability Index (Cpk) values using the steps and few examples given here.

Learn to Calculate Process Capability Index - Tutorial, Definition and Example

Process Capability (Cp) Definition:

Process capability is a technique to find out the measurable property of a process to a specification. Generally, the final solution of the process capability is specified either in the form of calculations or histograms

Process Capability Index (Cpk) Definition:

Process capability index (cpk) is the measure of process capability. It shows how closely a process is able to produce the output to its overall specifications.

Formula :

Where,

USL = Upper Specification Limit, LSL = Lower Specification Limit.

Example :

Food served at a restaurant should be between 38°C and 49°C when it is delivered to the customer. The process used to keep the food at the correct temperature has a process standard deviation of 2°C and the mean value for these temperature is 40. What is the process capability of the process?

Given,

USL (Upper Specification Limit) =49°C LSL (Lower Specification Limit) =39°C Standard Deviation =2°C Mean = 40

To Find,

Process Capability & Process Capability Index

Solution:
Process Capability :
Process Capability = (49 - 39) / (6 * 2)
= 10 / 12
= 0.833
Process Capability Index :
Solution 1
(USL-mean/ 3*std.Dev) = (49 - 40) / (3 * 2)
= 1.5
Solution 2
(mean-LSL/3*std.Dev) = (40 - 39) / (3 * 2)
= 0.166
Now, find the minimum value.
Process Capability Index = min (Solution 1, Solution 2)
= min (1.5 , 0.166)
= 0.166

Cp and Cpk are used in Six Sigma Quality Methods for analyzing the performance of the process carried out to deliver any product.

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