A process capability index is also based on the process standard
deviation, which is denoted as r. In practice, the parameter
r is unknown, and the use of an estimator is required in this
situation. Traditionally, the technique used for the estimation
of r consists in selecting m samples with the same size n. Simple
random sampling without replacement is the most common
sampling design used to select the various samples. Note that
the m samples must be obtained when it is known that the process
is stable. The information collected from these samples is
used for the purpose of estimating r. The most common
estimators used to estimate the process standard deviation
are based on the sample standard deviations and the sample
ranges