The Inertial Properties of Quality Control Charts

Many types of control charts have an ability to detect process changes that can weaken over time depending on the past data observed. This is often referred to as the “inertia problem.” We propose a new measure of inertia, the signal resistance, to be the largest standardized deviation from target not leading to an immediate out-of-control signal. We calculate the signal resistance values for several types of univariate and multivariate charts. Our conclusions support the recommendation that Shewhart limits should be used with exponentially weighted moving average charts, especially when the smoothing parameter is small.