Abstract :
[en] Recent studies have shown that enhancing the common T2 control chart by using variable sample sizes (VSS) and
variable sample intervals (VSI) sampling policies with a double warning line scheme (DWL) yields improvements in
shift detection times over either pure VSI or VSS schemes in detecting almost all shifts in the process mean. In this
paper, we look at this problem from an economical perspective, certainly at least as an important criterion as shift
detection time if one considers what occurs in the industry today. Our method is to first construct a cost model to
find the economic statistical design (ESD) of the DWL T2 control chart using the general model of Lorenzen and Vance
(Technometrics 1986; 28:3–11). Subsequently, we find the values of the chart parameters which minimize the cost
model using a genetic algorithm optimization method. Cost comparisons of Fixed ratio sampling, VSI, VSS, VSIVSS
with DWL, and multivariate exponentially weighted moving average (MEWMA) charts are made, which indicate the
economic efficacy of using either VSIVSS with DWL or MEWMA charts in practice if cost minimization is of interest to
the control chart user.
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