Using economically designed Shewhart and adaptive ―X charts for monitoring the quality of tiles

This paper presents the economic design of X control charts for monitoring a critical stage of the main production process at a tile manufacturer in Greece. Two types of X-charts are developed: a chart of the Shewhart type with fixed parameters and adaptive charts with variable sampling intervals and/or sample size. Our prime motivation was to improve the statistical control scheme employed for monitoring an important quality characteristic of the process with the objective of minimizing the relevant costs. At the same time we test and confirm the applicability of the theoretical models supporting the economic design of control charts with fixed and variable parameters in a practical situation. We also evaluate the economic benefits of moving from the broadly used static charts to the application of the more flexible and effective adaptive control charts. The main result of our study is that by redesigning the currently employed Shewhart chart using economic criteria, the quality-related cost is expected to decrease by approximately 50% without increasing the implementation complexity. Monitoring the process by means of an adaptive X-chart with variable sampling intervals will increase the expected cost savings by about 10% compared to the economically designed Shewhart chart at the expense of some implementation difficulty.