Real versus nominal value

The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present. Often a nominal value is de facto rather than an exact, typical, or average measurement.

In measurement, a nominal value is often a value existing in name only; it is assigned as a convenient designation rather than calculated by data analysis or following usual rounding methods. The use of nominal values can be based on de facto standards or some technical standards.

All real measurements have some variation depending on the accuracy and precision of the test method and the measurement uncertainty. The use of reported values often involves engineering tolerances.

One way to consider this is that the real value often has the characteristics of an irrational number. In real-world measuring situations, improving the measurement technique will eventually begin yielding unpredictable least significant digits. For example, a 1-inch long gauge block will measure to be exactly 1 inch long until the measuring techniques reach a certain degree of precision. As techniques improve beyond this threshold, it will become clear that 1 inch is not the real value of the gauge block length, but some other number approximates it.

In various subfields of engineering, a nominal value is one for which the "name" for the value is close to, but not the same as, the actual value. Some examples:

Other cases involve diameter, speed, and volume.

Sometimes the word "nominal" is misused in engineering contexts as a synonym for "normal" or "expected"; for example, The rotor resistances on all the other operating wheels are nominal.

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