Stability Index

[#1]

I am using the Process Capability platform and see in several places a "Stability Index" is calculated.  But I cannot find in the help documentation a definition for how Stability Index is calculated.  Can anyone help?

[#2 · Accepted solution]

The Process Screening Report - Stability Index 

[#3]

Just be sure to understand how Overall and Within Sigma are calculated.  

Stability Index = (Overall Sigma / Within Sigma)

Stability Ratio = (Overall Sigma / Within Sigma)^2

Overall Sigma is straightforward and assumes the data belongs to one big subgroup.  The sigma is calculated in relation to the grand mean of the data. The sum of squared distances from each observation to the grand mean.  Divided by the total number of observations.  

Within Sigma is not so straightforward and depends on the principles of rational sampling and rational subgrouping.   It can be calculated many different ways and that depends on the subgrouping.  Does it it make sense to subgroup the data in this way and if so why? 

If we subgroup "unlike things" together, then we will confuse within subgroup variation with between subgroup variation - we don't want this because then chart limits don't help us detect a meaningful signal.  So we have to think about things like: processing history (sequence of manufacture) and whether or not parts come from the same oven(s) or are made within the same hour(s).

[#4 · Accepted solution]

Here is a cut and paste from a search of the help pages for "stability index calculation"

Stability Index

A measure of stability of the process. A stable process has a stability index near one. Higher values indicate less stability. The stability index is defined as follows:

(Overall Sigma / Within Sigma)

If a three way control chart is selected in the launch, the stability index is defined as follows:

(Overall Sigma / Between-and-Within Sigma)

[#5]

Thank you!

[#6]

Stability Index of >1 could be an indicator of potential special cause variation in the process. I recommend testing the observations for stability violation before conducting capability study.