As variable control charts are capable of determining process capability of both types, i.e. instantaneous variability, and variability across time, y are used, when data for process capability analysis is collected in different shifts.

The two R?s of measurement systems capability are ?Reproducibility? and ?Repeatability?. They play very important role in measurement system capability studies.

C_pk is one sided PCR for specification limit nearest to standard deviation of process.

As control charts address issue of statistical control of process, y ensure that re must be random patterns in process output, so that variability is reduced.

In design of experiments, controllable inputs of a process are varied, and output of process is observed simultaneously, n a relationship between inputs and outputs, is made.

C_pk always shows actual capability as by interpretation of it, we can sense potential improvement possible in process by centering it.

The estimate of C_pm, when standard values of ?, ? are not given, is calculated by expression,
\(\widehat{C_{pm}}=\frac{C_p}{\sqrt{1+V^2}}\)

The one sided process capability ratio that uses USL of quality characteristic is written as,
\(C_{pu}=\frac{USL-?}{3?}\)

Two processes having different centering can have a same C_pk. So we need to evaluate C_p to check equality of PCR C_pk with it. This way we get perfect centering between specifications.

The minimum value of two sided specifications PCR C_p for an existing process is generally taken as 1.33. For one sided specifications, PCR C_p is taken as 1.25.