Showing papers by "William H. Woodall published in 1990"
TL;DR: In this article, the authors evaluate the properties of an exponentially weighted moving average (EWMA) control scheme used to monitor the mean of a normally distributed process that may experience shifts away from the target value.
Abstract: Roberts (1959) first introduced the exponentially weighted moving average (EWMA) control scheme. Using simulation to evaluate its properties, he showed that the EWMA is useful for detecting small shifts in the mean of a process. The recognition that an EWMA control scheme can be represented as a Markov chain allows its properties to be evaluated more easily and completely than has previously been done. In this article, we evaluate the properties of an EWMA control scheme used to monitor the mean of a normally distributed process that may experience shifts away from the target value. A design procedure for EWMA control schemes is given. Parameter values not commonly used in the literature are shown to be useful for detecting small shifts in a process. In addition, several enhancements to EWMA control schemes are considered. These include a fast initial response feature that makes the EWMA control scheme more sensitive to start-up problems, a combined Shewhart EWMA that provides protection against both larg...
1,380 citations
TL;DR: The variable sampling interval (VSI) CUSUM chart as mentioned in this paper uses short sampling intervals if there is an indication that the process mean may have shifted and long sampling intervals when there is no indication of a change in the mean.
Abstract: A standard cumulative sum (CUSUM) chart for controlling the process mean takes samples from the process at fixed-length sampling intervals and uses a control statistic based on a cumulative sum of differences between the sample means and the target value. This article proposes a modification of the standard CUSUM scheme that varies the time intervals between samples depending on the value of the CUSUM control statistic. The variable sampling interval (VSI) CUSUM chart uses short sampling intervals if there is an indication that the process mean may have shifted and long sampling intervals if there is no indication of a change in the mean. If the CUSUM statistic actually enters the signal region, then the VSI CUSUM chart signals in the same manner as the standard CUSUM chart. A Markov-chain approach is used to evaluate properties such as the average time to signal and the average number of samples to signal. Results show that the proposed VSI CUSUM chart is considerably more efficient than the standard CUS...
229 citations
TL;DR: In this article, the average run lengths of the zone control chart are compared with that of several Shewhart charts with and without runs rules, and it is shown that the standard Zone Control Chart has performance similar to some even simpler charts and a much higher false alarm rate than the She Whithart chart with all of the common runs rules.
Abstract: Average run lengths of the zone control chart are presented, The performance of this chart is compared with that of several Shewhart charts with and without runs rules, It is shown that the standard zone control chart has performance similar to some even simpler charts and a much higher false alarm rate than the Shewhart chart with all of the common runs rules. It is also shown that a slightly modified zone control chart outperforms the Shewhart chart with the common runs rules.
41 citations
TL;DR: In this paper, a FORTRAN program is given for evaluating the properties of the run length distribution of a Shewhart quality control chart with supplementary runs rules, and the program has two parts.
Abstract: A FORTRAN program is given for evaluating the properties of the run length distribution of a Shewhart quality control chart with supplementary runs rules. The program has two parts. The first part (Part 1) of the program determines the minimal set of st..
32 citations