On monitoring process variability under double sampling scheme

The presence of variation in all manufacturing and measurement processes is a natural phenomenon and is the key factor which affects the performance of all types of processes. A better understanding of the causes of variability in any processes is necessary to improve the process. For an efficient monitoring of process variability, we have suggested a set of variance type control charts based on auxiliary characteristics and evaluated their performances in terms of Average Time to Signal (ATS) (the performance measure at every point of variability shift) and Average Extra Quadratic Loss (AEQL) (the performance measure over the whole process shift range) under normal and gamma process environments. We have also examined the effects of contaminated environments on the ATS performance of different variance based charting structures. Illustrative examples on some selective variance type control structures are also provided for procedural details. Finally we have closed with concluding remarks about this study.