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Accuracy and Precision

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Accuracy and Precision

In everyday speech, the terms accuracy and precision are frequently used interchangeably.  However, their scientific meanings are quite different.  Accuracy is a measure of how close a measurement is to the correct or accepted value of the quantity being measured.  Precision is a measure of how close a series of measurements are to one another.  Precise measurements are highly reproducible, even if the measurements are not near the correct value.

Darts thrown at a dartboard are helpful in illustrating accuracy and precision

A basketball game requires shooting accuracy

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

The distribution of darts on a dartboard shows the difference between accuracy and precision. [Figure1]

Assume that three darts are thrown at the dartboard, with the bulls-eye representing the true, or accepted, value of what is being measured.  A dart that hits the bulls-eye is highly accurate, whereas a dart that lands far away from the bulls-eye displays poor accuracy.  The Figure above demonstrates four possible outcomes.

  1. The darts have landed far from each other and far from the bulls-eye.  This grouping demonstrates measurements that are neither accurate, nor precise.
  2. The darts are close to one another, but far from the bulls-eye.  This grouping demonstrates measurements that are precise, but not accurate.  In a laboratory situation, high precision with low accuracy often results from a systematic error.  Either the measurer makes the same mistake repeatedly or the measuring tool is somehow flawed.  A poorly calibrated balance may give the same mass reading every time, but it will be far from the true mass of the object.
  3. The darts are not grouped very near to each other, but are generally centered around the bulls-eye.  This demonstrates poor precision, but fairly high accuracy.  This situation is not desirable in a lab situation because the “high” accuracy may simply be random chance and not a true indicator of good measuring skill.
  4. The darts are grouped together and have hit the bulls-eye.  This demonstrates high precision and high accuracy.  Scientists always strive to maximize both in their measurements.

Summary

  • Accuracy is a measure of how close a measurement is to the correct or accepted value of the quantity being measured.
  • Precision is a measure of how close a series of measurements are to one another.

Image Attributions

  1. [1]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0

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