Integration of quadratic loss function approach in the economic design of VSI control chart with combined attribute-variable inspection

This paper proposes a variable sampling intervalnpx-X over bar control chart for monitoring the process mean using combined attribute-variable inspection. An economic design of the proposed chart using Taguchi's quadratic loss function is presented. The proposed model minimizes the total expected cost per unit time while minimizing out-of-control average time-to-signal and maintaining reasonable in-control average time-to-signal. The numerical results indicate that, compared with the corresponding fixed sampling interval control chart scheme, the proposed control chart yields better performance in terms of both shorter time to signal a process shift and less total expected cost per unit time. Sensitivity analyses of the cost model are also carried out by using a fractional factorial experimental design.