The closeness of agreement between repeated measurements of the same quantity, indicating the repeatability of a sensor.Precision is a measure of how consistently a sensor or measurement procedure yields the same result when the same quantity is measured repeatedly under unchanged conditions. It is a statement about the spread or scatter of the data points around their average, not about whether that average is correct. A precise instrument produces tightly clustered readings, while an imprecise one produces widely scattered ones, even if the average of those readings happens to be accurate.
Precision is quantified statistically, typically as the standard deviation or variance of a set of repeated measurements. The smaller the standard deviation, the higher the precision. It is distinct from accuracy, which describes how close the average of the measurements is to the true value. A sensor can be precise but inaccurate (systematically offset) or accurate but imprecise (large random scatter). In practice, precision is limited by random noise sources, such as thermal fluctuations, electronic noise, or mechanical vibrations, which cause the sensor's output to vary from one trial to the next.
Precision is a key specification for all measurement instruments, from laboratory balances and thermometers to GPS receivers and particle detectors, where it determines the smallest resolvable difference in the measured quantity.
In metrology, the term 'precision' is sometimes replaced by 'repeatability' to emphasize that it refers to the agreement between results obtained under the same conditions, not the closeness to a true value.