The MaxMin EWMA tolerance limits

When there is a change in a process, the MaxMin exponentially weighted moving average (EWMA) control chart shows which parameters have increased or decreased. The MaxMin EWMA may also be viewed as smoothed tolerance limits. Tolerance limits are limits that include a specific proportion of the population at a given confidence level. In the context of process control, they are used to make sure that production will not be outside specifications. In this article, we provide useful coverages for the MaxMin EWMA chart, when also used as tolerance limits. The proposed EWMA smoothed tolerance limits require relatively small sample sizes to attain useful coverages at high confidence levels. The MaxMin EWMA chart has already been successfully field-tested and subsequently implemented with 100 multi-stream processes.