A CONTROL CHART FOR PRELIMINARY-ANALYSIS OF INDIVIDUAL OBSERVATIONS

A preliminary control chart based on individual observations for detec ting a shift in either the mean or variance or both is developed from a likelihood ratio test. The advantages over use of the X control char t and moving range control chart are improved detection of sustained s hifts, in addition to improved diagnostic information indicating multi ple shifts and whether an out-of-control signal is primarily due to a shift in the mean or variance or a combination. Approximate control li mits, determined from simulation, are given for the proposed control c hart.