A time-varying EWMA-CV control chart based on Upper Triangular Minimax ranked set sampling method

When the process mean fluctuates with time and process standard deviation is a linear function of process mean, the coefficient of variation is usually used to measure the process variation in industry. On the other hand, the use of Ranked Set Sampling as a smart sampling technique enables the ranking of samples before measurement. The purpose of this research is to introduce a new sampling method referred to as Upper Triangular Minimax Ranked Set Sampling and design a time-varying Exponentially Weighted Moving Average control chart to monitor the coefficient of variation. Also, in this research, average run length, standard deviation of run length, median of run length, extra quadratic loss, relative average run length and performance comparison index criteria under two states including zero-state and steady-state have been used to compare the performance of the designed control chart with the ones provided in the literature. The results show the superiority of the proposed control chart based on the novel sampling method rather than the control charts available in the literature. The application of the proposed control chart is also shown through a real case and compared with available competitors in the literature.