Authors: Shashibhushan B. Mahadik
Addresses: Department of Statistics, Solapur University, Solapur, 413255, India
Abstract: The idea of variable warning limit is applied to variable sampling interval (VSI) Hotelling's T2 charts. The purpose is to reduce the average frequency of switches between the sampling interval lengths of VSI T2 charts during their implementation. Expressions for the performance measures for variable sampling interval and warning limit (VSIWL) T2 charts are developed. The performances of VSIWL T2 charts are compared numerically with that of static, VSI, and VSI (1, 3) T2 charts, where VSI (1, 3) T2 charts are the VSI T2 charts with runs rule (1, 3) for switching between sampling interval lengths. A VSIWL T2 chart yields different out-of-control performances for the same in-control statistical performance depending on the choices of values of its warning limit. The values of its warning limit can be chosen to yield the average frequency of switches between its sampling interval lengths dramatically and significantly smaller than that of VSI and VSI (1, 3) T2 charts, respectively, and to yield slightly better statistical performance than that of VSI and VSI (1, 3) T2 charts.
Keywords: adaptive control charts; average number of samples to signal; average number of switches to signal; multivariate SPC; statistical process control; steady-state average time to signal; variable warning limits; variable sampling intervals; VSI; performance measures.
International Journal of Quality Engineering and Technology, 2013 Vol.3 No.4, pp.289 - 302
Received: 05 Dec 2012
Accepted: 21 Dec 2012
Published online: 12 Aug 2013 *