Software tip: Lloyd Nelson control limit option

Steve DaumCustomers often request that we add very specific statistical features to our software products. Before we decide to implement new features, we consider several questions. Is the feature within our realm of core competency? Is the feature related to the types of analysis we already do? Will the feature appeal to a narrow or wide swath of our customers? Is the feature aligned with our mission of providing the simplest solutions that apply practical statistics to improve our customers’ quality?

If the feature makes it though this gauntlet, we study the specifics of the new feature. What algorithms are required? What formulae will be used? How will the feature be tested? Where will the feature be visible in the user interface? This process takes time. We are continuously improving our turn around on feature requests, but many steps exist between getting the idea from the customer into a working product.

A good example of this is process is the Lloyd Nelson Limits option. The idea behind this feature was suggested by Lloyd Nelson in the Technical Aids section of The Journal of Quality Technology (Vol. 14, No. 3, Pg. 172, July 1982). Lloyd Provost and his colleagues at Associates in Process Improvement (API) promoted this method of calculating control limits. API, creators of The Model for Improvement for healthcare organizations, provided guidance in implementing this feature in our quality improvement charting software CHARTrunner.

The new control limit option is designed to improve the usefulness of individuals control charts. In some cases, out-of-control conditions on the associated moving range chart can affect the control limit values on the individuals chart. The result can be control limits that are wider, and therefore less sensitive, than they should be. When the Lloyd Nelson Limit option is selected, these out-of-control conditions are removed when calculating control limits on the individuals chart. The resulting control limits make it more likely that users of the chart will detect assignable cause variation in their process. This feature is available in the current version of CHARTrunner.