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Characteristics of A Measurement

In making a measurement , one of the first considerations is the resolution of the measuring instrument. Resolution refers to the minimum change in value that the instrument can reliably indicate. For instance , many dial gages are graduated in 0.001�0.01mm)divisions , but can be read to 0.0005�0.005mm). If it is necessary to measure a dimension to only 0.005�0.15mm), there is no advantage in using an instrument with a resolution to 0.0005�0.015mm); As a matter of fact, it is likely to be a disadvantage. There is considerable confusion between the terms repeatability and accuracy as they relate to measurement . Often , too, repeatability and resolution are equated. Accuracy means closeness to truth . In discussing the accuracy of a micrometer or an electronic gage, reference is to the degree to which it can measure the true size of a part. The first requirement of any measuring system is that it have adequate repeatability . For instance , if widely varying results are obtained with each measure of the length of a gage block, these results are meaningless , and the measuring system is ineffective. The difference between repeatability and accuracy is illustrated by the target analogy, a group of measurements accurate , and vice versa.