can be used to determine whether the process was in control when both estimates were calculated they use simulation to obtain the approximate 90th 95th 99th and 99.9th percentiles of the distribution of F for sample sizes m =10(5)100, assuming that the process is on control since this is an empirical reference distribution observed values of f that excess one the these percentiles is an indication that the process was not in control over the time period during which the m observation were collected