Discovering critical qualities or sets of qualities (hereafter referred to as just qualities) and authorizing them as criteria for determining product Quality requires not only that they be identified precisely, but also that an optimum value (or goal) be established that represents the optimum, desired product Quality. Additionally, tolerances from the optimum goals are established as limits for acceptable product Quality.

The tolerances may be above the goal, below the goal or around the goal. If a particular quality is given a specific value as being required for optimum product Quality, then any change in the value of that quality will effect the product Quality according to equation [1].

Improvement of quality is discussed extensively in industry (see Kaizen Philosophy), but as it relates to reality - there are limits to improving some qualities, as the cost sometimes becomes too great or the improvement is not perceptible. Consequently, part of the setting of goals is cost related, which depends on the product, market, technology and corporate dictates.

Using this concept Variability can be defined as the totality of all variations in all the qualities:



Quality is inversely proportional to the variation of all qualities, which are pertinent to the product (or service or process). Product Quality now becomes quantifiable, analyzable, predictable, controllable and improvable. Selection of these qualities and their appropriate variations and optimum goals takes time to think through; but when established, Quality for the product is defined. Later a more technical definition will be shown - one that engineers and designers can actually use for calculating a figure of merit for Quality. But before we do that, the three Quality categories need a bit more explanation so that it is clear where the various qualities fit within the product development activity.