Science 122 Accuracy vs. Precision

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Science 122


Lab 5

The Science of Measurement: Accuracy vs. Precision

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The dictionary definitions of these two words do not clearly make the distinction as it is used in the science of measurement.

Accurate means "capable of providing a correct reading or measurement." In physical science it means 'correct'. A measurement is accurate if it correctly reflects the size of the thing being measured.

Precise means "exact, as in performance, execution, or amount. "In physical science it means "repeatable, reliable, getting the same measurement each time."

We can never make a perfect measurement. The best we can do is to come as close as possible within the limitations of the measuring instruments.

Let's use a model to demonstrate the difference.

Suppose you are aiming at a target, trying to hit the bull's eye (the center of the target) with each of five darts. Here are some representative pattern of darts in the target.

Neither Precise Nor Accurate

This is a randomlike pattern, neither precise nor accurate. The darts are not clustered together and are not near the bull's eye.

Precise, Not Accurate

This is a precise pattern, but not accurate. The darts are clustered together but did not hit the intended mark.

Accurate, Not Precise

This is an accurate pattern, but not precise. The darts are not clustered, but their 'average' position is the center of the bull's eye.

Precise and Accurate

This pattern is both precise and accurate. The darts are tightly clustered and their average position is the center of the bull's eye.


Does that help to understand the distinction between accuracy and precision as the terms are used in the science of measurement?

If not, think about the patterns. In each case did you hit the target where you aimed? Which one represents 'repeatability or reliability'? Which one represents 'correct but sloppy'?

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