1.4 Formulate Critical-to-Quality Characteristics
CTQs (Critical-to-Quality) are the key measurable characteristics of a product or process whose performance standards or specification limits must be met to satisfy the VOCs (Figure 1-3). They align improvement or design efforts with customer requirements.
Figure 1-3 CTQ: An engineering metric.
CTQs represent the product or service characteristics defined by the customer (internal or external). They can include the upper- and lower-specification limits or any other factors related to the product or service. A CTQ item usually must be determined from a qualitative customer statement and "translated" into an actionable, quantitative business specification.
In robust design, a Critical-to-Quality characteristic should be related to the perceived result. In a robust design experiment, the measured output of the system is the CTQ. The CTQ performance is frequently inconsistent with the ideal perceived result because of noise. When determining what to measure in an experiment—the CTQ—first consider the customer's perspective of system functionality.
For example, customers use their brakes with the intent of slowing down or stopping the car. The ideal perceived result is for the car to smoothly slow down or come to a stop every time the brakes are applied. With this in mind, the team must now determine which portion of the brake system to focus on and establish a CTQ, in engineering metrics, that quantifies the functionality of that system.
For nonrobust brake systems, the perceived result is often conveyed in terms that describe unintended results, such as noisy or rough. Optimizing a CTQ related to brake functionality—the distance to stop—minimizes unintended results. This philosophy represents a change in the way the engineer approaches the design process.
In sum, a CTQ is an engineering metric that quantifies the system's functionality (i.e., its ability to meet the VOCs). Critical-to-Quality characteristics drive efforts to control energy transformation within a product or system.