The Role of Specification Limits
They are primarily for interactions with customers and management.
It is still surprising how many people, from engineers to managers to quality professionals to technicians, possess limited understanding of product and process (manufacturing) limits.
These limits, applied to numerical measurements of key product quality characteristics, often drive behavior and actions as well as frustration for users and decision-makers. Operations can become dysfunctional when this lack of understanding produces further tightening of limits.
In this age of automation, the range of manufacturing parameters can reach an exponential number each day so wasted time and resources can incur through a lack of understanding, and mismanagement of limits can be significant! We want to focus on three limits and their relationship to each other. Future columns will discuss control limits and disposition limits but for now we will center on specification limits.
The differences between specification limits and control limits have been widely discussed since Walter Shewhart of Western Electric invented the process control chart in the mid-1920s. However, these differences seem to be poorly understood at all levels at many organizations and even among some quality professionals.
Product engineers, quality professionals and manufacturing personnel, from managers to technicians, still focus on specification limits as the most important limits associated with any individual quality measurement. To the engineer and the quality professional, if a product falls within specification limits, the item must be fully functional or as Dr. Joseph M. Juran said, the product is “fit for use.” To the manufacturing personnel, this just means the product is assumed to be ‘ok to ship’ to customers which is always their desired result.
Of the three relevant limits, it might be surprising to realize that specification limits often will be considered the least important to actual manufacturing operations and decision-making processes. This assumes, however, the specification limits are properly defined, understood and managed, which is a highly questionable assumption, even for large operations.
Let’s define specification limits. The general definition is limits within which a product would be expected to perform its stated and intended function for customer use. Specification limits, therefore, are related to product design. They should be set in the product design phase and effectively fixed for manufacture.
It might be surprising that some but not all organizations recognize this definition of specification limits. In one such organization the practice was to apply specification limits to averages of process results, thereby failing to acknowledge the application of specification limits to individual product units. Imagine the surprise when the new quality manager asked for the latest Cpk results and noticed Cpk values less than one!
This organization failed to understand that the variation of average results used to set its specification limits would be significantly less than the variation of individual product unit measurements for use in calculating Cpk. The organization as a whole, including the quality group, failed to have a necessary understanding of the central limit theorem which is one of the most important theorems in statistics.
In this case properly defined specifications were of limited value to manufacturing operational decision-making. Essentially this organization had been managing specification limits as disposition limits (which will be discussed in a later column), so it had not fully recognized the need for separate limits.
If specification limits do not play the primary and direct role in decision-making, what is their value? While they may not play a direct role in process control limits within the manufacturing environment, specification limits do facilitate determination of useful product disposition limits. Additionally, they can even play a role in determining required sensitivity levels in setting process control limits.
Furthermore, specification limits are primarily for interactions with customers and management. Also, they are very useful in the calculation of Cpk statistics commonly requested by customers and periodically used by managers. I realize that some quality professionals and other experts discount the value of this statistic but with my many decades of experience I’ve found it to be extremely valuable to evaluate how well processes are managed.
While this alone makes them important and of long-term value to manufacturing operations, specification limits are not generally thought to be of significant value in routing daily manufacturing endeavors. Mostly operations’ personnel look primarily at process limits and pay little attention to specification limits but we’ll address that in a later column.
Perhaps you work for an organization that effectively knows and manages the limits it applies to routinely acquired measurements of key product quality characteristics. If so, you are truly fortunate to work in an enlightened atmosphere. In my career I certainly benefited in that many of our key management personnel were ASQ CQEs so there was ample understanding of product and process control limits.