PRECISION AND ACCURACY
The performance of a measuring instrument is represented by the terms precision and accuracy. A good instrument must be precise and accurate.
PRECISION
The precision of an instrument is the extent to which the instrument repeats its result while making repeat measurements on the same unit of product. It is the repeatability of the measuring process. It refers to the repeat measurement for the same unit of product under identical condition. It indicates to what extent the identically performed measurement agrees with each other. If the instrument is not precise it will give widely varying results for the same dimension when measured again and again. The set of observations will scatter about the mean. The scatter of these measurements is designated as (= the standard deviation) it is used as an index of precision. The less the scattering the more precise is the measurement. Thus lower the value of the more precise is the measurement.
ACCURACY
Accuracy of an instrument is the extent to which the average of a long series of repeat measurements made on the same unit of a product differs from the true value of the product. The difference between the true value and the measured value is known as the error of measurement.
It is practically difficult to measure exactly the true value. Therefore a set of observations is made whose mean value is taken as the true value of the quality measured.