... process control is non- normal, traditional Shewhart variable charts may not be applicable If the performance of the SPC scheme is adversely affected by a violation of the assumptions, modification of ... for non- normal data They showed that, in the non- normal case, a properly designed EWMA control chart will have an in -control ARL that is reasonably close to the value of 370.4 for the individuals ... in -control and out-ofcontrol ARLs 2.2.2 Variable Charts Recall that traditional Shewhart type variable control charting methods are often based on two assumptions: that sample observations are...