Accuracy and precision
In science, engineering, industry and statistics, accuracy is the degree of conformity of a measured or calculated quantity to its actual, nominal, or some other reference, value. Precision characterises the degree of mutual agreement among a series of individual measurements, values, or results.
In a common analogy illustrating the difference between accuracy and precision, repeated measurements are compared to arrows that are fired at a target. Accuracy describes the closeness of arrows to the bullseye at the target center. Arrows that strike closer to the bullseye are considered more accurate. The closer a system's measurements to the accepted value, the more accurate the system is considered to be.
To continue the analogy, precision would be size of the arrow cluster. When all arrows are grouped tightly together, the cluster is considered precise since they all struck close to the same spot, if not necessarily near the bullseye. The measurements are precise, though not necessarily accurate.
Ideally a measurement device is both accurate and precise, with measurements all close to and tightly clustered around the known value.
The accuracy and precision of a measurement process is usually established by repeatedly measuring some traceable reference standard. Such standards are defined in the International System of Units and maintained by national standards organisations such as the National Institute of Standards and Technology.
Accuracy is characterised as the difference between the mean of the measurements and the reference value, the bias. Establishing and correcting for bias is necessary for calibration.
Precision is usually characterised in terms of the standard deviation of the measurements, sometimes called the measurement process's standard error.
Precision is sometimes stratified into:
A useful analogy
Quantifying accuracy and precision
