- The measurement value (or measurement) is the value given by a measuring instrument
- The true value is the actual value of the property being measured
A measurement value does not specify a single exact value, it specifies a range of values within which the true value is believed to lie. This range of values is called the uncertainty interval or (simply the uncertainty).
A measurement is expressed using the value at the midpoint of the uncertainty interval (nominal value) along with the difference either side to the limits of the uncertainty interval e.g. 10 +/- 0.5 cm
- Relative uncertainty is the size of the uncertainty compared to the size of the nominal value.
The smaller the relative uncertainty is the more precise the measurement is.
- Error is the difference between the measured value and the true value i.e. the difference between the true value and the furthest limit of the uncertainty interval.
Relative error is the size of the error compared to the size of the nominal value.
The smaller the relative error is the more accurate the measurement is
- The accuracy and precision required for a measurement value depends on the particular application.