What Is the Probability That Your Process Is Predictable?

How to compute a p-value for your process behavior chart

Story update 1/15/2019: Thanks to the sharp eye of Dr. Stan Alekman, who spotted an inconsistent value in figure 2, I discovered an error in the program used to construct the table of critical values for the prediction ratio. I have now corrected that problem and updated the entries in the table in figure 2. If you previously downloaded this column, you might want to download the corrected version below.

Software packages use p-values to report the results of many statistical procedures. As a result some people have come to expect a p-value as the outcome of any statistical analysis. This column will tell you how to compute and use a p-value for a process behavior chart.

…

Want to continue?

By logging in you agree to receive communication from Quality Digest. Privacy Policy.

Create a FREE account

Forgot My Password

Comments

How to compute a p-value for your process behavior chart

Hi Dr. Wheeler,

We used this back in my days at Sylvania. It still calculates on my Excel charts. We called it the P.S.C. (Process Shift Coefficient). Isn't it virtually the same as Shewhart's original Test 2? I don't have my copy of "Economic Control of Quality of Manufactured Product" handy, but I remember crediting him with the concept. Of course, neither he nor we went so far as to calculate a P value if I remember correctly, but we did use f tests to determine significance of the value. As you point out, no single summary statistic is as valuable as looking at and understanding the chart, but it comes in handy when you need to quantify instability. Not everyone takes the time to understand the charts.

Doug A.