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Practices of Science: Precision vs. Accuracy

NGSS Science and Engineering Practices:

 

Precision and accuracy are two ways that scientists think about error. Accuracy refers to how close a measurement is to the true or accepted value. Precision refers to how close measurements of the same item are to each other. Precision is independent of accuracy. That means it is possible to be very precise but not very accurate, and it is also possible to be accurate without being precise. The best quality scientific observations are both accurate and precise.
 
A classic way of demonstrating the difference between precision and accuracy is with a dartboard. Think of the bulls-eye (center) of a dartboard as the true value. The closer darts land to the bulls-eye, the more accurate they are. 
  • If the darts are neither close to the bulls-eye, nor close to each other, there is neither accuracy, nor precision (SF Fig. 1.5 A).  
  • If all of the darts land very close together, but far from the bulls-eye, there is precision, but not accuracy (SF Fig. 1.5 B).   
  • If the darts are all about an equal distance from and spaced equally around the bulls-eye there is mathematical accuracy because the average of the darts is in the bulls-eye. This represents data that is accurate, but not precise (SF Fig. 1.5 C). However, if you were actually playing darts this would not count as a bulls-eye!
  • If the darts land close to the bulls-eye and close together, there is both accuracy and precision (SF Fig. 1.5 D). 
 
<p><strong>SF Fig. 1.5.</strong> Dartboards showing different accuracy and precision scenarios.</p><br />
 
Question Set: 
  1. An oceanographer needs to go out in a boat to collect an important temperature and salinity data logger that is attached to an underwater buoy. How does each of the following situations illustrate the differences between precision and accuracy?
    1. The oceanographer checks the weather forecast the night before her trip so she knows what to wear on the boat. The TV forecaster says it will be between 26 and 31 degrees (°) Celsius (C) at noon the next day. The actual temperature reading the next day on the boat at noon is 28° C. 
    2. When the oceanographer’s Global Positioning System (GPS) indicates that she is at the location of the underwater buoy, she anchors the boat and jumps in the water to collect the data logger. However, she can’t see the buoy. The other GPS units belonging to her colleagues on the boat also indicate that they are at the correct location. After an extensive search, the oceanographer finds the buoy 50 meters (m) from the boat. 
    3. While on the way back to shore, the oceanographer throws in a fishing line to see if she can catch anything for dinner. She is lucky enough to catch a mahi-mahi. When she pulls it out of the water, her colleagues estimate the weight of the fish. Their estimates are 16.1 kilograms (kg), 16.8 kg, and 15.9 kg. When they weigh the fish upon returning to shore, the actual weight is 18.2 kg.
       
  2. Write your own scenario illustrating the difference between accuracy and precision. Swap your scenario with a classmate. Identify your classmate’s scenario measurements as accurate or inaccurate and precise or imprecise.
     
  3. A dart player can see how accurate his or her dart throws are by comparing the location of the thrown darts to the target, the bulls-eye of the dartboard.
    1. How is this model different from scientists who are measuring a natural phenomenon? 
    2. Is there a way for scientists to determine how accurate their measurements are? Explain your answer

 

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.