Study on Asymmetrical Range Charts

The range chart of National Standard is symmetrical and based on "3σ" principle of normal distribution. We proposed the idea of asymmetrical range chart based on the statistical characteristics of range in this article, constructed three types of asymmetrical range charts and worked out their coefficients of upper and lower control limits at the alert rate of 0.27%. In practice, engineers can use these results very easily. These three types of asymmetrical range charts have their own characters, which were introduced in this article. We also compared them with the range chart of National Standard in the respects of power function and Average Run Length (ARL). We can make the alert rate of the three types of asymmetrical range charts reach 0.27%, which is much lower than the one of range chart of National Standard. Unbiased control chart is the most attractive because of the good characteristic of unbias.