Cp, Cpk vs Pp, Ppk

In the last post we looked at the different process capability indices like Cp, Cpk, Cpm etc. But there is also Pp and Ppk indices that measure process performance. There is a fundamental difference between the two types of indices.
We will now try to understand the difference between process capability and process performance.
And when to use what capability index?

Process capability measures the ability of the process to consistently produce output within specification limits. Process capability indices are calculated assuming the process is statistically stable i.e. has only common cause variation.

Process Performance is an estimate of the process capability during initial setup. The indices are calculated assuming the process is unstable. Such a process has both common and special cause variation. A comparison between process capability and process performance helps understand the difference between the two.
 

 
Have you used Cp, Cpk or Pp, Ppk for measuring your process capability? We would love to hear from you at info@sybeq.com.

Histogram & capability analysis

In the last post we looked at tools for analyzing distribution of variable data. Histogram is the most popular distribution analysis tool. Using the histogram we can also predict the capability of the process.

Histogram shows the frequency distribution of the data. The curve shows shape of the distribution. Skew measures asymmetry of the distribution. A positive skew indicates a histogram with a longer right side tail whereas a negative skew has a longer left side tail.

So where is the tie in between the histogram and capability analysis?
Process capability analysis compares process performance against specifications. Histogram shows the distribution of process data. If the histogram graphically showed the specification limits then it is showing if the process is performing within specs or not. So visually the histogram would show if the process is producing good parts or not.

Numerically capability of a process can be analyzed by calculating the capability indices. There are several capability indices each showing an aspect of process capability. Most popular capability indices are Cp, Cpk, Pp, Ppk.

Read our White Paper on different capability indices and their uses.

What capability indices do you to determine process capability? How do you use the histogram for distribution analysis?