Accuracy and precision are used in context of measurement. Accuracy refers to the degree of conformity and correctness of something when compared to a true or absolute value, while precision refers to a state of strict exactness — how consistently something is strictly exact. In other words, the precision of an experiment, object, or value is a measure of the reliability of, for example, an experiment. The accuracy of an experiment, object, or value is a measurement of how closely results agree with a true or accepted value.
Both accuracy and precision are terms used in the fields of science, engineering, and statistics.
Contents: Accuracy vs Precision
One can say that a measurement is accurate but not precise, precise but not accurate, or neither or both. An example of bad precision with good accuracy might be a lab refrigerator that holds a constant temperature of 38.0F. A temperature sensor is tested 10 times in the refrigerator. The temperatures from the test yield the temperatures of: 37.8, 38.3, 38.1, 38.0, 37.6, 38.2, 38.0, 38.0, 37.4, 38.3. This distribution shows no impressive tendency toward a particular value (lack of precision) but each value does come close to the actual temperature (high accuracy).
edit Target Comparison
Accuracy is the degree of correctness, while precision is how strict that correctness is (or isn't) — how reproducible results are. For this example, consider results from a round of target practice.
Arrows are fired at a target, and measurements are taken in relation to the bull's eye at the center of the target. Accuracy describes how close the arrows are to the bull's-eye. The closer an arrow is to the bull's eye, the more accurate the shot.
How precise the shots are depends on how often the arrows land near each other on the target. When all or most arrows are grouped tightly together, the shots fired can be considered precise since they all landed near the same spot, if not necessarily near the bull's-eye. This is how results can indicate precision but not necessarily accuracy. However, it is important to note that it is not possible to reliably achieve accuracy without precision.
edit Number of Measurements
One measurement is enough to determine accuracy, but at least several measurements are required to determine precision. In the example above, one arrow alone can show how accurate an archer can be, but only with several shots fired can one know how precise the archer is.
While a precise measurement may speak highly of an instrument's quality, an accurate reading will not reflect on the quality. Accuracy is an agreement of a measured value with an expected value. For example, a stopped clock will be accurate twice in day, but it will not be precise — i.e., able to consistently and accurately keep time throughout the day. In the case of a clock, how precisely it measures time matters a great deal and determines quality.