On Valid Signals in Joint Schemes for the Process Mean and Variance

Abstract

Abstract Joint schemes for the process mean and the variance are essential to determine if unusual variation in the location and spread of a quality characteristic occurred. This paper comprises a systematic study on the phenomena of misleading, unambiguous and simultaneous signals while dealing with Shewhart and EWMA joint schemes for the process mean and the variance of a normally distributed quality characteristic. Examples have been added to illustrate how the R statistical software can be used to assess the performance of joint schemes in practice, namely when it comes to the occurrence of those valid signals.