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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.
==Measurement==
In measurement, a nominal value is often a value existing in name only;〔ASTM D3039, D4139, and others〕 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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