4.1.1 Accuracy and Precision

In conversational English, the terms accuracy and precision are virtually identical in meaning. Roget's Thesaurus ^[1] lists these words as synonyms and Webster's Dictionary ^[2] gives almost identical definitions for them. However, these terms are defined very differently in engineering textbooks ^[3]- ^[5]. Combining the definitions from these and other sources gives us an idea of the accepted technical meaning of the words:

Accuracy - The difference between the average of measurements and a standard sample for which the "true" value is known. The degree of conformance of a test instrument to absolute standards, usually expressed as a percentage of reading or a percentage of measurement range (full scale).

Precision - The variation of a measurement system obtained by repeating measurements on the same sample back-to-back using the same measurement conditions.

According to these definitions, precision refers only to the repeatability of a series of measurements. It does not refer to consistent errors in the measurements. A series of measurements can be incorrect by 2 V, but as long as they are consistently wrong by the same amount, then the measurements are considered to be precise.

This definition of precision is somewhat counterintuitive to most people, since the words precision and accuracy are so often used synonymously. Few of us would be impressed by a "precision" voltmeter exhibiting a consistent 2-V error! Fortunately, the word repeatability is far more commonly used in the test engineering...