Precision and
accuracy are two terms used to describe groups of measurements, usually in scientific or mathematical situations. Precision describes how close measurements are to each other, whereas accuracy describes how close measurements are to the
accepted measurement.
Example: A pencil is exactly 5.6 inches long.
1. Measurement set A: 4.1 in., 7.3 in. Neither precise nor accurate.
2. Measurement set B: 6.5 in., 6.6 in. Precise, but inaccurate.
3. Measurement set C: 5.7 in., 5.6 in. Precise and accurate.
Lord Brawl asked, "What about accurate, but imprecise?" This condition can't logically exist. For a set of data to be accurate, they must all be close to the accepted value, therefore they would be precise as well.
Another example of the usage of the terms precise and accurate, as pointed out by
rootbeer277, is
target shooting. If, for instance, you throw three
darts, the outcomes could be as follows:
1. The darts are all clustered very closely around the
bullseye. Your throws are both accurate and precise.
2. The darts are all clustered closely together, but not near the center of the target. Your throws are precise, but inaccurate.
3. The darts are scattered around the dartboard in no particular pattern. Your throws are neither precise nor accurate.
Keep in mind that these terms are both
relative; there is no definition for how close a set of measurements must be to be precise, and no definition for how close the measurements must be to the accepted value to be accurate.
See also Precision of a Measuring Tool.