It is customary to express the accuracy of a measurement in terms of the error that can be expected. The error is defined as the difference between the observed value of the measurement (OV) and the true or accurate value (TV). Thus, if the error is designated ?, then

An error can be expressed in actual units of measurement or as a fraction or percentage of the true value. An error can be positive or negative, depending on whether the observed value is greater or less than the true value.

Sometimes statements about the accuracy of measurements tend to get sloppy. Such a statement might be "Orifice meters are 3% accurate." This statement could be interpreted incorrectly in either of two ways. First, the statement that the meter is 3% accurate implies that it could be 97% in error, which is unrealistic. Second is the implication that no matter what the reading of the meter might be, it will be in error by 3%. This is not realistic either because most industrial meters can usually be calibrated so that they are accurate at at least two points in their range of measurement. A proper statement of accuracy for the meter is "This meter is accurate to within 3% (of the true value)." The key word is "within."