The Specifications Approach to Variation

In the section on variation, we said that some degree of variation is inevitable in every process. The traditional approach to this problem has been to attempt to establish limits within which variation is acceptable. So, for example, the dimensions of a manufactured part will be specified in terms of a nominal or target value, and tolerance limits above and below the nominal. Parts with dimensions inside the specified tolerance are deemed to be acceptable, and parts with dimensions outside the tolerance limits are declared unacceptable, or non-conforming.

Unfortunately, specifications do not address the nature of a process. They are simply a way of separating the output of the process. Each piece produces is either within spec or outside spec: a good part or a bad one. This has several serious implications, including:

• Focus on the product and not the process leads to an uneconomical cycle of inspection followed by rework or scrap.
• Conformance to specifications means relying solely on results measures and ignoring process measures.

Let's look at these points in a little more detail.