Scientist and engineers compare a measurement to the accepted value using the term accuracy. The measurement system must provide both accuracy and precision if it is valid.
Accuracy relates closely to precision but is not the same. A common example includes the use of a target and darts. Four darts, with even spacing, around the bull's eye are accurate, yet not precise. Two darts with relatively close spacing yet far from the center of the target demonstrate precision but not accuracy. Accuracy and precision involve all darts landing near the bull's eye.
Scientists often express accuracy by means of significant figures. Convention dictates that the accuracy of the measurement is within one-half of the last significant figure. The accepted value results from repeated measurements of a traceable standard. The International System of Units defines these standards. In the United States, the National Institute of Standards and Technology maintains such traceable standards.
Increasing the number of measurements and averaging the results improves the accuracy of the measurement but does not necessarily improve the precision. In medicine and psychological studies, the scientist increases the number of measurements by increasing the number of participants in a study. However, increasing participants also increases the number of variables, some of which the researcher has not considered, into the study.