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Keywords:

  • adaptive control chart;
  • average number of samples to signal;
  • average number of observations to signal;
  • average number of switches to signal;
  • statistical process control;
  • steady-state average time to signal

The notion of variable warning limits is proposed for variable sample size and sampling interval (VSSI) inline imagecharts. The basic purpose is to lower down the frequency of switches between the pairs of values of the sample sizes and sampling interval lengths of VSSI inline image charts during their implementations. Expressions for performance measures for the variable sample size, sampling interval, and warning limits (VSSIWL) inline image charts are developed. The performances of these charts are compared numerically with that of VSSI and VSSI (1, 3) inline image charts, where VSSI (1, 3) inline image charts are the VSSI inline image charts with runs rule (1, 3) for switching between the pairs of values of sample sizes and sampling interval lengths. Runs rule (1, 3) greatly reduces the frequency of the switches; however, it slightly worsens the statistical performances of the VSSI inline image charts in detecting moderate shifts in the process mean. It is observed that the out-of-control statistical performance and overall switching rate of VSSIWL inline image charts are adaptive for the same in-control statistical performances. These charts can be set to yield exactly similar performances as that of VSSI (1, 3) inline image charts, to yield tradeoff performances between that of VSSI (1, 3) and VSSI inline image charts, or to yield significantly lower switching rate than even that of VSSI (1, 3) inline image charts at the cost of slightly inferior statistical performances than that of VSSI (1, 3) inline image charts. Copyright © 2012 John Wiley & Sons, Ltd.