Accuracy refers to the degree of agreement of the measured dimension with its true magnitude, while Precision stands for degree to which an instrument gives repeated measurement of the same standard. In general, good accuracy not necessarily means good precision, and vice versa. For example, the following figure illustrates the results of a series of the measurements for a part. It can be seen that the measuring instrument has small random error, and thus good precision. On the other hand, the result also shows that this measuring system has poor accuracy, because there is a large difference between the true value and the measured values of the part. This situation also signals the system has a system error.

Figure: Accuracy and Precision of a measuring system