Many people know about William Tell shooting the apple on his son’s head. What can this archery event teach us about accuracy and precision? What do these words mean? When are they equal, and when are they not? “Accuracy,” “precision” and “resolution” are sometimes misunderstood or misrepresented. A measurement can be precise yet inaccurate, regardless of its resolution. And the resolution of a measurement may have nothing to do with how many places there are after the decimal point. How can that be?

ISO standards and most of the standards labs around the world—NIST, the National Physical Laboratory, Physikalisch-Technische Bundesanstalt, etc. —provide definitions of these important words, and there are many contextual definitions that are specific to devices or situations. I’m not going to accurately match any of those definitions. I’ll just explain them as concepts with application examples.

First, it’s important to understand the differences in these words. Many people use them interchangeably, as if they have the same meaning. They don’t. Even if you know the definitions, there may be more to them than meets the eye. Let’s start with the definition of "accuracy."

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