How can cp be greater than one

In a Six Sigma process, the Cpk equals 2. The Cpk is inversely proportional to the standard deviation, or variability, of a process.

The higher the Cpk, the narrower the process distribution as compared with the specification limits, and the more uniform the product. As the standard deviation increases, the Cpk index decreases. At the same time, the potential to create product outside the specification limits increases.

Cpk can only have positive values. It will equal zero when the actual process average matches or falls outside one of the specification limits. The Cpk index can never be greater than the Cp, only equal to it. This happens when the actual process average falls in the middle of the specification limits.

He has over 23 years of industrial experience in engineering, quality and process improvement and has served at the executive level as Corporate Vice President of Quality for Flextronics International. During his tenure, he institutionalised and standardised the application of statistical methods and Six Sigma in Motorola's worldwide manufacturing, production and engineering operations.

What if the specifications of the product is less than the Minimum Specification limit or if it is greater than the Maximum standard limit?

Cpk is considered zero in that case. Negative values by formula are just replaced by zero indicating that the process is not capable at all.

This is true to best of my knowledge. The allowable spread is the difference between the upper and lower specification limits. The actual spread is 6 times the estimated standard deviation. Plus or minus 3 times the estimated standard deviation contains A Cp of one indicates that the width of the process and the width of the specification are the same.

A Cp of less than one indicates that the process spread is greater than the specification. This means that some of the data lies outside the specification.

The standard deviation for Cpm is based on using the target value rather than the mean which will make sigma pm larger and Cpm smaller when the process is not centered on the target value. However, Cp might only be 0. Can I compare two processes based on only the Cpk values of each of them? Is there any other tool by which I can say that one process is better than other?

It depends on what you mean by better. If the processes are producing the same product dimension, then you can compare them more or less directly. Cpk includes a centering factor as well as the variation factor. Unless you want to compare centering as part of the two processes, use Cp. Can the process performance index Ppk be applied on the ongoing process?

If yes, how? The capability indices are designed to be applied to ongoing processes. They are an indication of what a customer can expect in terms of quality from a particular process.

If you have a control chart on a characteristic for a process, SQCpack will calculate these values for you if you enter the specifications. If you do not have the software, the capability analysis article series provides information on calculating capability.

Why would I have Cp and Cpk indices well over 1 when some readings are outside the specification limits? My first guess would be that if you look at a control chart of the data, it is out of control.

Before you can do capability analysis, the process should be predictable and that requires that it be stable in-control. For a more detailed discussion, see How can Cpk be good with data outside the specification? Quality Advisor A free online reference for statistical process control, process capability analysis, measurement systems analysis, control chart interpretation, and other quality metrics.