<meta http-equiv="refresh" content="1; url=/nojavascript/">

# Accuracy and Precision

Data that is close to a measured value versus data that is grouped closely together.
%
Progress
Practice Accuracy and Precision
Progress
%
Accuracy and Precision

Credit: Daniel Arizpe
Source: http://commons.wikimedia.org/wiki/File:High_school_basketball_game.jpg
License: CC BY-NC 3.0

#### How do professional basketball players improve their shooting accuracy?

Basketball is one of those sports where you need to hit the target.  A football field goal kicker might have room for some deviation from a straight line – for college and pro football there is an 18 foot 6 inch space for the ball to go through.  In basketball, the basket is only 18 inches across and the ball is a little less than 10 inches across – not much room for error.  The ball has to be on target in order to go into the basket and score.

### 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

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. [Figure2]

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.

Credit: Image copyright zebrik, 2012
Source: http://www.shutterstock.com
License: CC BY-NC 3.0

Students in a chemistry lab are making careful measurements with a series of volumetric flasks. Accuracy and precision are critical in every experiment. [Figure3]

#### 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.

#### Practice

Take the quiz at the link below:

#### Review

Questions

1. Define accuracy.
2. Define precision.
3. What can be said about the reproducibility of precise values?

### Image Attributions

1. [1]^ Credit: Daniel Arizpe; Source: http://commons.wikimedia.org/wiki/File:High_school_basketball_game.jpg; License: CC BY-NC 3.0
2. [2]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
3. [3]^ Credit: Image copyright zebrik, 2012; Source: http://www.shutterstock.com; License: CC BY-NC 3.0

Please wait...
Please wait...