Accuracy, Precision and Uncertainty in Measurement

Physics is fundamentally a science built on observation and experiment: whenever we study a physical system — for example, during an atmospheric study, where we might measure atmospheric pressure, wind velocity, humidity, and more — the numbers we record are observations of various physical quantities, and each such observation needs to be judged by two separate, independent criteria: accuracy and precision.

Accuracy is how close a measured value is to the actual (true) value of the quantity being measured. A measurement is accurate if it agrees with reality.

Precision is a different property: it describes how close multiple measurements of the same quantity are to each other — that is, whether repeated measurements are reproducible, giving nearly identical results each time. A measurement can be precise (highly reproducible) without being accurate (if all the repeated readings are consistently offset from the true value by some systematic error), and equally a measurement can occasionally land close to the true value (accurate on that one occasion) without being precise (if repeated attempts scatter widely). In any real experiment, a set of observations may turn out to be both accurate and precise, or neither, or only one of the two — and the goal of a careful experimenter is always to achieve measurements that are both accurate and precise. …