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Abstract
A new form of policy for the design of np charts is presented. The policy is characterized by a variable sampling interval, such that the taking of each sample is begun at a constant prior probability that the process is out of control. The new policy is shown to generate a vector of sampling intervals which dominates the fixed sampling interval under assumed known system parameters. The desireability of the VSI approach is shown to depend on the power of the sampling procedure, contradicting previously published results stating that a VSI procedure should always use the shortest and longest intervals possible. The vector of sampling intervals generated by the new policy converges to a fixed interval policy when conditions are such that variable sampling intervals are not warranted.
