A conventional X control chart works in an open loop manner. That is, the process control engineer waits until the control scheme signals an out-of-control condition. This paper proposes a combine...

A Combined Adaptive Sample Size and Sampling Interval X Control Scheme

This survey presents, in a comprehensive way, the substantial recent developments in the statistical and economic design of adaptive or dynamic charts that allow some of their parameters to change ...

A Survey of Recent Developments in the Design of Adaptive Control Charts

Recent theoretical studies have shown that the X-bar chart with variable sampling intervals (VSI) and the X-bar chart with variable sample size (VSS) are quicker than the traditional X-bar chart in detecting shifts in the process. This article considers..

X̄ Chart with Variable Sample Size and Sampling Intervals

Varying the parameters of the X chart has been explored extensively in recent years. In this paper, we extend the study of the X chart with variable parameters to include variable action limits. ...

X̄ Charts with Variable Parameters

Traditional control charts for process monitoring are based on taking samples of fixed size from the process using a fixed sampling interval. Variable Sample Size (VSS) and Variable Sampling Interval (VSI) control charts vary the sampling rate from the process as a function of the data from the process. VSS and VSI control charts sample at a higher rate when there is evidence of a change in the process, and are thus able to detect process changes faster than traditional control charts. This paper considers general VSS and VSI control charts and develops integral equation and Markov chain methods for finding the statistical properties of these charts. EWMA charts with the VSS and/or the VSI features are studied in detail, and different ways of defining the EWMA control statistic are investigated. It is shown that using either the VSS or VSI feature in an EWMA control chart substantially improves the ability to detect all but very large shifts in the process mean. The VSI feature usually gives more...

EWMA control charts with variable sample sizes and variable sampling intervals

Standard Shewhart control charts employ fixed sample sizes at equal sampling intervals. By varying the sample size depending on the current location of the process mean, the mean time to detect an off-target condition can be reduced. The adaptive-sample-size control chart is compared with the fixed-sample-size control chart in terms of average run length under shifts in the process mean of variable magnitude. Significant improvements have been obtained with the adaptive-sample-size charts, particularly for small shifts. These improvements are achieved without increasing the in-control average sample size beyond that of the fixed-sample-size approach. A fast initial response is suggested and advantages of the procedure over fixed-sample-size control are illustrated with two examples from discrete manufacturing processes.

S. S. Prabhu

Papers

X¯ chart with adaptive sample sizes