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Sunday, August 25, 2013

Precision vs. Accuracy



It is useful to understand the meaning of the terms “precision and accuracy” when relating to measurement instrumentation. Precision relates to the ability of an instrument to very closely repeat measurements time after time. The term “accuracy” relates to the ability of an instrument to measure values which are very close to the correct value as determined by an industry accepted standard or authority. As can be seen from the image above, instruments can have any of four combinations of precision and accuracy

a) the upper left hand target depicts an instrument which has both low precision (poor reproducibility) and low accuracy (agrees poorly with the correct value); 

b) the lower left hand target depicts an instrument which has excellent repeatability but for which all of its measurements are in error by approximately the same amount; 

c) the upper right hand target depicts an instrument which exhibits poor repeatability, but the average of its measurements is very close to the correct value; and 

d) the lower right hand target depicts an instrument which is very repeatable and for which the average of its readings agrees closely to the correct value.

This clarification of terms is often helpful when communicating with coworkers or customers when discussing data.