Four words that are not synonyms · The three pillars: repeatability, predictability, predictive capability

1.1 Four words that are not synonyms

In everyday speech “precise” and “accurate” are used interchangeably. In shooting — as in metrology, from which shooting borrows the concepts — they name two different and independent properties.

Shooting is precise when the shots fall close to one another: the group is tight, the system behaves the same way every time. Shooting is accurate when the mean point of impact coincides with the point of aim — when, in technical terms, the MPI (mean point of impact) coincides with the POA (point of aim). The two can exist separately, and Figure 1.1 shows the four possible combinations: a tight group off center is precise and not accurate; a wide group whose center falls on the point of aim is accurate and not precise. It is worth saying at once in which sense the book uses the two words, because metrology divides them more finely: there “accuracy” is the umbrella term, and the closeness of the mean value to the reference goes by the name of trueness, following the three-way split of ISO 5725 cited among the sources in Appendix A. Here, as on the range, accuracy is the centering of the mean point and precision is the tightness of the group.