and S2, where S is the
standard deviation of a normal process are used. The optimal
Cusum schemes to detect small and large increases in the
variability of a normal process are designed. The control
statistic S2 is then used to show that the Cusum scheme is
superior to the exponentially weighted moving average (EWMA) in
terms of its ability to quickly detect any large or small
increases in the variability of a normal process. It is also
shown that Cusum with control statistics sample variance (S2)
and sample standard deviation (S) perform uniformly better
than those with control statistic logS2. Fast initial response (FIR) Cusum properties are also presented.">