Control charts applying a sequential test at fixed sampling intervals

Traditional control charts take samples from the process using a fixed sample size and fixed sampling interval. The sampling rate in a control chart, however, can be varied depending on what is observed from the process in order to reduce the average time required to detect process changes. A fixed sampling interval control chart is proposed which varies the sampling rate at each sampling point as a function of the data observed at the current sampling point, by sampling sequentially, in order to monitor the mean of a normal quality characteristic. Integral equations are used to develop expressions for statistical properties of the proposed chart. It is shown that the proposed chart has administrative advantages and detects most shifts in the process mean substantially faster than the traditional (X) over bar and cumulative sum (CUSUM) charts and other variable sampling rate versions of these charts. Guidelines are given for the design of the proposed chart.