An economic design of variable sampling interval (X)over-bar control charts

This paper considers an economic design of variable sampling interval (X) over bar control charts in which the sampling interval between two successive sampling points is varied based on the value of the preceding sample mean. A cost model is constructed which involves the cost of false alarms, the cost of detecting and eliminating an assignable cause, the cost associated with production in out-of-control state and the cost of sampling and testing. A method of finding optimal values of sample size, sampling interval lengths, control limits and threshold limits to select one of the sampling interval lengths is presented. Variable and fixed sampling interval (X) over bar control charts are compared with respect to the expected cost per unit time.