Trueness, precision and accuracy: a critical overview of the concepts as well as proposals for revision

Many parameters are defined by international scientific committees to describe the performances of a measuring system functioning according to a given measurement procedure. A careful inspection of several international and national documents in matters of measurement reveals inconsistencies in the definition of some fundamental terms. This paper discusses the meaning currently attributed to trueness, precision and accuracy assuming as a reference the International Vocabulary of Metrology 2012 (VIM). We support that accuracy cannot be intended as composed by trueness and precision (as declared by various standards of international – as ISO – or national organisms), since trueness and precision requires large number (infinite number, according to VIM 2012) of replicate measured quantity values to be assessed while accuracy refers to a single measured quantity value, according to widespread current definitions. As to VIM and other scientific organisms, accuracy can be intended only in a qualitative fashion, avoiding to associate numbers with it. Hence, a measurement result unbiased and precise is accurate. We propose to intend the term trueness only in an ideal meaning and to introduce the concept of exactness correctly describing the matching between a measurement result  calculated from a large number of replicate measured quantity values  and an accepted reference quantity value. The range of variability of a single result of measurement can be assessed by way of the measurement uncertainty, which can be quantified by constructing the uncertainty budget and cannot neither be considered nor used as an expression of accuracy.