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 preced ing sample mean. A cost model is constructed which involves the cost o f false alarms, the cost of detecting and eliminating an assignable ca use, the cost associated with production in out-of-control state and t he 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. Va riable and fixed sampling interval (X) over bar control charts are com pared with respect to the expected cost per unit time.