An Improved Variable Sample Size And Sampling Interval S Control Chart
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Date
2018-05
Authors
See, May Yen
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
The standard deviation chart or S chart is used to monitor the process
standard deviation. In this thesis, some of the features of the S chart are altered to
increase the chart’s sensitivity towards small and moderate shifts in the process
standard deviation. This new chart for monitoring the process standard deviation is
known as the improved variable sample size and sampling interval ( t VSSI ) S chart.
The design and construction of the t VSSI S chart is based on the idea of the
improved variable sample size and sampling interval ( t VSSI ) X chart proposed by
Noorossana et al. in 2016. The t VSSI X chart improves the efficiency of the
X-type charts in detecting small and moderate shifts in the process mean. The
t VSSI S chart uses two sample sizes and three sampling intervals. The Markov
chain approach is employed in the design of the t VSSI S chart for evaluating the
chart’s average time to signal (ATS) and expected average time to signal (EATS)
performances. The ATS and EATS performances of the optimal t VSSI S chart are
compared with that of the optimal exponentially weighted moving average (EWMA)
S, optimal modified variable sample size and sampling interval (VSSI) S and
standard S charts. The ATS and EATS values of all the charts are computed using the
MATLAB programs. The simulation programs in the Statistical Analysis System
(SAS) software are also used to verify the accuracy of the ATS and EATS values
computed via MATLAB. It is found that the t VSSI S chart is superior to the
standard S, modified VSSI S and EWMA S charts, in detecting moderate and large
shifts in the process standard deviation, in terms of the ATS and EATS criteria. An
illustrative example is given to show the construction of the t VSSI S chart.
Description
Keywords
The standard deviation chart or S chart , monitor the process standard deviation