Editing Statistical process control (section)

====Process stability metrics====

When monitoring many processes with control charts, it is sometimes useful to calculate quantitative measures of the stability of the processes. These metrics can then be used to identify/prioritize the processes that are most in need of corrective actions. These metrics can also be viewed as supplementing the traditional [[process capability]] metrics. Several metrics have been proposed, as described in Ramirez and Runger.<ref name="Ramarez2006">{{cite journal
| last1 = Ramirez |first1=B.
| last2 = Runger |first2=G.
|title= Quantitative Techniques to Evaluate Process Stability
|journal= Quality Engineering
|volume=18
|issue=1
|pages=53–68
|year=2006 | doi = 10.1080/08982110500403581
|s2cid=109601393
}}</ref>
They are (1) a Stability Ratio which compares the long-term variability to the short-term variability, (2) an ANOVA Test which compares the within-subgroup variation to the between-subgroup variation, and (3) an Instability Ratio which compares the number of subgroups that have one or more violations of the [[Western Electric rules]] to the total number of subgroups.