A new nonparametric EWMA sign control chart

Dear Editor, I have recently read the paper by Yang, Lin, and Cheng (2011), published in your journal. In this paper the authors proposed the nonparametric EWMA sign and Arcsine control charts for efficient monitoring of the deviation from process target in the variables data. There are some issues with the proposed nonparametric EWMA sign chart that needs to be corrected. The EWMA statistic used for the EWMA sign chart is given as – see Eq. (1) of Yang et al. (2011, p. 6240) EWMAMi 1⁄4 kMi þ ð1 kÞEWMAMi 1 ð1Þ