Quite simply, d²σ is an abbreviation for “double-digit sigma.” It refers to a process with a mean that is at least 10σ from the nearest spec limit.

The use of the term “double-digit” stems from early attempts to quantify and communicate achievable and sustainable levels of performance. In searching for a description of performance results, we found that a single “maximum” number was not sufficient, since we have often exceeded 10σ quality levels, and have occasionally gone far beyond even that level of performance. The best way to describe the group of techniques that we employ, and the results we achieve, is “double-digit sigma” or “d²σ” in shortened form.