Tolerance limits as a continuous assessment tool

Non-parametric tolerance limits are used to make sure that production will not be outside specifications for non-normal populations. They are constructed usually at the beginning of a process run. These limits do not reflect changes of tolerance level over the lifetime of the process. This article proposes the continuous assessment of process tolerance. The procedure utilizes the exponentially weighted moving average method and provides evidence of process tolerance over the lifetime of the process. The resulting tolerance limits are shown to be narrower on average and exhibit considerably lower variability than the non-parametric limits commonly used. The sample size required for these tolerance limits can also be reduced by a considerable amount over time.