Precision is about scatter: how small the spread of repeat readings on one item is under a defined set of conditions. It is expressed numerically as a standard deviation, variance or coefficient of variation, and it is always tied to conditions: repeatability, intermediate precision or reproducibility. Precision says nothing about whether the readings cluster around the right value.
Precision is great, five readings within a micron, but they're all 6 microns off the master.
Do not mix it up with
- Accuracy: Accuracy asks whether readings land near the true value; precision only asks whether they land near each other.
Related terms
Accuracy
How close a measured value comes to the true value, a qualitative idea rather than a reported number.
Repeatability
How tightly readings bunch when one person repeats a measurement with the same setup, in one place, in one short sitting.
Reproducibility
How well results on the same item agree when the lab, operator, equipment or day is changed on purpose.
Intermediate precision
Precision measured within one lab over longer periods, with changes such as different days, operators or recalibrations.
Standard deviation
The usual measure of scatter in a set of values, the square root of the average squared deviation from the mean.